Desk Reference for hematology - part 8 docx

The abnormal clone expands withinthe bone marrow, probably due to decreased NK-cell activity compared with normal cells,thus replacing the hematopoietic stem cell pool and giving rise to

PAROXYSMAL COLD HEMOGLOBINURIA 691

PAPPENHEIMER BODIES

Basophilic red blood cell inclusions, often in small clusters near the periphery of the cell.They are composed of ferritin aggregates, or of mitochondria or phagosomes containingaggregated ferritin They often occur in reticulocytes The associated disorders include:

Splenectomy (post)

Sideroblastic anemia

Lead toxicity

PARAPROTEINEMIA

See Monoclonal gammopathies

PARIETAL CELL ANTIBODIES

See Gastric disorders

PAROXYSMAL COLD HEMOGLOBINURIA

(PCH) A very rare form of cold autoimmune hemolytic anemia characterized by acuteepisodes of massive hemolysis following cold exposure The disease was frequently diag-nosed during the latter half of the 19th century because of its supposed association withcongenital or tertiary syphilis Now PCH occurs as an acute febrile illness associated withviral syndromes, particularly the childhood exanthems There is usually one self-limitedattack of acute intravascular hemolysis with hemoglobinuria The prognosis is good Achronic form of the disorder is characterized by recurrent episodes of hemolysis precipi-tated by exposure to cold temperature

The cause of autoantibody production in PCH is unknown There are no known racial

or genetic predispositions During severe chilling, blood flowing through skin capillaries

is exposed to low temperatures The antibody (see Donath-Landsteiner test) is biphasic,and early-acting complement components bind to red blood cells at lowered temperatures.Upon return of the cells to 37°C in the central circulation, the cells are rapidly lysed byactivation of the terminal complement sequence through C9 The Donath-Landsteinerantibody dissociates from the red blood cells at body temperature

Constitutional symptoms are prominent during the paroxysm After cold exposure, thepatient develops aching pains in the back or legs and abdominal cramps Chills and feverfollow Urine passed after onset of symptoms typically shows hemoglobinuria, which,with the general symptoms, lasts a few hours

The hemoglobin level can drop rapidly in a severe attack Chronic anemia, raised

reticulocyte count, hemoglobinemia, and hyperbilirubinemia may be present, depending

on the frequency and severity of attacks Complement titers are depressed during an acuteepisode because of consumption in the hemolytic reaction Spherocytes and erythro- phagocytosis by monocytes and neutrophils are typically found on the blood film during

an attack Leukopenia is seen early in the attack, followed by neutrophilia The urinemay be dark red or brown due to the presence of hemoglobin or methemoglobin.The direct antiglobulin (Coombs) test is positive during and following a paroxysm,but negative between attacks The positive reaction is due to coating of surviving red cellswith complement The Donath-Landsteiner antibody is a nonagglutinating IgG that binds

692 PAROXYSMAL NOCTURNAL HEMOGLOBINURIA

in the cold and readily dissociates at room temperature and above The antibody isdetected in vitro by the biphasic Donath-Landsteiner test In this test, the patient’s freshserum is incubated initially with red cells at 4°C and the mixture warmed to 37°C Intensehemolysis occurs Antibody titers rarely exceed 1:16 The Donath-Landsteiner antibodyusually has specificity for the P blood group antigen This is a unique IgG complement-activating antibody that hemolizes P1 and P2 red cells but not p or Pk cells The anti-Pautoantibody usually binds at temperatures below 20°C If the cells are exposed to anti-

P at low temperatures in the presence of complement, C1q attaches to the membrane, and

if the suspension is warmed to 25°C or higher, the complement activity concludes withhemolysis

PCH must be distinguished from chronic cold-agglutinin disease, which manifestsepisodic hemolysis and hemoglobinuria, a distinction made primarily in the laboratory.Patients with PCH lack high titers of cold agglutinins Other disorders with similar clinicalpresentation are distinguished by history and by appropriate serologic studies

Acute attacks can be prevented by avoidance of cold Further treatment is rarely sary If for some reason red blood cell transfusion is required, P-antigen-positive cellswould have to be used due to the rarity of P-antigen-negative donors Blood should betransfused using an in-line blood warmer at 37°C and keeping the patient warm Mostpatients with chronic idiopathic PCH survive for many years despite occasional paroxysms

neces-of hemolysis Splenectomy is not likely to be of help, but plasmapheresis may temporarilyreduce hemolysis

PAROXYSMAL NOCTURNAL HEMOGLOBINURIA

(PNH) An acquired intrinsic red blood cell defect due to hematopoietic stem cell deficiency

of glycosylphosphidylinositol (GPI) It is postulated that an environmental toxin induces

a mutation of PIG-A gene of hematopoietic stem cells The abnormal clone expands withinthe bone marrow, probably due to decreased NK-cell activity compared with normal cells,thus replacing the hematopoietic stem cell pool and giving rise to aplastic anemia, dys-erythropoiesis (myelodysplasia), or to PNH, where red blood cells, and to a lesser extentgranulocytes and platelets, have a deficiency of surface proteins, leading to complement-mediated lysis.406,407

Pathophysiology

The characteristic defect of increased sensitivity of red blood cells to complement-mediatedlysis, either by the classical or by the alternative pathways This can be precipitated byvarious factors:

Lowering of pH (acidified serum test)

Reduction in ionic strength (sucrose lysis test)

Coating of red blood cells with antibody such as anti-A

Increase in magnesium concentration

Treatment with cobra venom

Previously, it was thought that lowering of blood pH during sleep explained nocturnalhemoglobinuria, but this is disputed

Several red blood cell membrane proteins attached to GPI are deficient in PNH, e.g.,leukocyte alkaline phosphatase, acetyl cholinesterase, urokinase plasminogen activator,

PAROXYSMAL NOCTURNAL HEMOGLOBINURIA 693

and several other proteins that regulate complement function The hereditary absence ofCD59 antigen in these patients is critical in producing clinical PNH with a significantdegree of hemolysis The conversion of N-acetylglucosamine and glucosamine-phospho-inositol to monolipids is defective, so that GPI cannot be manufactured normally.Absence of “decay accelerating factor” (DAF/CD55) — which acts to accelerate thedestruction of erythrocyte-bound C3 convertase, leading to its action on surface-boundC3d being amplified — was thought by some to be the basic pathologic lesion Granulocytesand platelets, and possibly lymphocytes, also show increased complement-mediated lysis.Occasionally the abnormal clone disappears completely, but transition to an acute leu-kemia, though rare, has been well documented

Clinical Features

Although the classical passage of dark brown urine first thing in the morning occurs inonly a minority of patients, all show clinical or laboratory signs of a chronic intravascular hemolysis This can be initiated by:

Episodes of infection

Strenuous exercise

Surgery

Injection of radiological contrast dyes

Transfusion of whole blood

Free plasma hemoglobin absorbs nitric oxide that is essential for smooth-muscle tion Iron lost in the urine as hemosiderin or hemoglobin results in iron deficiency, whichmay be exaggerated by gastrointestinal tract bleeding associated with severe thrombocy- topenia from bone marrow aplasia or dyserythropoiesis Treatment with oral iron hasbeen known to exacerbate the hemolysis, mainly because the iron raises erythrocyteoutput, including PNH cells This added hemolysis is not clinically significant

func-Symptoms during a paroxysm are caused by disturbances of smooth-muscle function.They include abdominal pain of a colicky nature, and there may be abdominal tenderness,dysphagia, erectile failure, and severe lethargy Renal tract manifestations include hypo-posthenuria, abnormal tubular function, and impaired creatinine clearance, though sig-nificant renal impairment is very rare Severe headaches and pain in the eyes are common.The most serious complication is venous thromboembolic disease due to platelet GPIdeficiency and activation by complement There is a predilection for intra-abdominal veins.Hepatic vein thrombosis (Budd-Chiari syndrome) carries a particularly poor prognosis.Microthrombi in the pulmonary vasculature results occasionally in pulmonary hyperten-sion Thrombosis of major cerebral vessels is rare, but in some the headaches are due tothrombosis of small vessels in the cerebral cortex Pregnancy with PNH carries a high risk

of severe thrombotic complications Anticoagulation throughout pregnancy may berequired

The clinical course is variable Median survival is 10 years, the common causes ofmorbidity and mortality being thrombosis and bone marrow failure

Laboratory Features

Anemia of variable severity occurs in most, but not all, patients, and is usually associatedwith a mild-to-moderately raised reticulocyte count This may be associated either with

694 PARVOVIRUS

a macrocytic anemia or with myelodysplasia (MDS), but if there is severe iron deficiency,

there will be a microcytic anemia

Neutropenia and thrombocytopenia are usually present, but seldom to a severe degree

The bone marrow usually shows a variable degree of erythroid hyperplasia, but it may

be hypoplastic or show dyserythropoietic changes In iron-deficient patients, stainable

iron is not present

Hemoglobinuria occurs in a minority of patients, but hemosiderinuria is usually

demonstrable

Screening tests are the acidified serum test and the sucrose lysis tests Confirmation of

diagnosis is demonstration of a deficiency of GPI-linked molecules on the surface of

hematopoietic cells by flow cytometry demonstration of immunophenotypes CD14+,

CD16+, CD24+, CD55+, and CD59+

Management

Supportive care with red blood cell transfusion is often needed Theoretically, washed

red blood cells should be used to avoid transfusing complement, but unwashed packed

red blood cells are probably equally satisfactory

Iron deficiency should be corrected by attention to the source of red blood cell loss as

well as giving oral iron supplements

Lifelong prophylactic oral anticoagulation with warfarin is needed for those patients

experiencing recurrent venous thromboses Treatment of the Budd-Chiari syndrome is by

thrombolytic therapy followed by oral anticoagulants Because of the high risk of

throm-bosis, where possible, elective surgery should be avoided

Androgens and glucocorticoids have been used with mixed results and are probably

only useful as short-term measures in patients with aplastic anemia, where they

occasion-ally result in improvement of peripheral-blood counts Erythropoietin may benefit a

minority of patients Trials of complement inhibitor eculzimab are in progress; preliminary

results show excellent control of hemolysis Allogeneic stem cell transplantation is

poten-tially curative in younger patients, though overall results have been rather disappointing

compared with those obtained for uncomplicated aplastic anemia

PARVOVIRUS

Direct infection of erythroid precursors by a small, nonenveloped single-stranded DNA

virus consisting of 5500 nucleotides A particular strain, parvovirus B19, is pathogenic to

humans and causes a variety of different diseases (see Table 125) Their lack of envelope

and small DNA content render them extremely stable to heat and lipid solvents, and hence

they are readily transmitted by blood transfusion and coagulation factor concentrates (see

Transfusion transmitted infection) Immunoglobulin (Ig) G and IgM can be detected by

enzyme-linked immunosorbent assay and immunofluorescence

TABLE 125

Disorders Induced by Parvovirus B19 Infection

Fifth disease a

Polyarthropathy Pure red cell aplasia Hydrops fetalis Transient erythroblastopenia of childhood

a Also called erythema infectiosum or “slapped cheek” disease.

P BLOOD GROUPS 695

About 50% of the population has serological evidence of past infection petent patients develop acute self-limiting illnesses, or even asymptomatic seroconversion,and no specific therapy is required Those who have a chronic hemolytic anemia, e.g.,hereditary spherocytosis and sickle cell disease, may suffer from infections that cause

Immunocom-erythroid aplastic crisis Most patients have a spontaneous remission, but some have a persistent pure red cell aplasia Characteristically, giant pronormoblasts are found in the

bone marrow of acutely infected patients, with confirmation provided by detection ofspecific IgM or IgG in the serum and by a polymerase chain reaction (PCR) in patientsunable to mount an immune response

PATHOGEN-RECOGNITION MOLECULES

(PRM) Receptors on the surface of cells, particularly histiocytes (macrophages), that

recognize pathogens These can enter the cell and either become resident in the cytoplasm

or are destroyed

Toll-like receptors (TLRs), upon activation, induce intercellular signaling pathways thatactivate microcidal responses Other PRMs are nucleotide-binding oligomerizationdomain proteins (NODs) Genetic mutations affecting NODs are associated with Blau’s

syndrome (arthritis, skin rashes, and uveitis), Crohn’s disease (see Intestinal tract

disor-ders — chronic inflammatory disease), and sarcoidosis.

P BLOOD GROUPS

A specific antigen–antibody system arising from the P and globoside blood group systems

and the globoside collection of blood group antigens; located on red blood cells (RBCs),

lymphocytes , and monocytes (see Blood groups).

Biochemistry

Like the ABO(H), Lewis, and I blood group antigens, the P blood group antigens are

carbohydrates Production of the Pk, P, and P1 antigens is through different biosyntheticpathways: the P1 antigen has been assigned to the P blood group system, the P antigen

to the globoside system, and Pk and Luke (LKE) antigens to a separate “collection.” Pk and

P antigens are also detected on erythroblasts, fibroblasts, and vascular endothelium cells

Genetics and Phenotypes

Biosynthetic pathways are complex and incompletely understood The phenotypes aresummarized in Table 126 As with ABH antigens, genes code for glycosyl transferases thatcatalyze the transfer of monosaccharides onto carbohydrate precursors

TABLE 126

Phenotypes of the P and Associated Blood Group Systems

696 PEARSON-MARROW-PANCREAS SYNDROME

Antibodies and Their Clinical Significance

Anti-P1 is sometimes found in the plasma of P1 (P2) persons The antibody is almostalways naturally occurring and is of the IgM class Anti-P1 is sometimes errone-ously called anti-P (see below)

The apparent incidence of anti-P1 is dependent upon the pretransfusion testing ods in use, as most examples show activity only at temperatures below 37°C Attemperatures below 20°C, anti-P1 can be demonstrated in a high proportion ofpatients’ sera

meth-Anti-P1 only very rarely causes hemolytic blood transfusion complications if

incom-patible RBCs are transfused It can be ignored for the purposes of transfusion if

it is inactive at 37°C in direct agglutination or antiglobulin methods Provision ofblood for patients with anti-P1 should not be difficult

Anti-P1 does not cause hemolytic disease of the newborn (HDN) for the same reasons that ABO HDN is rare (see ABO (H) blood groups).

Anti-PP1Pk (anti-Tja) is regularly found in the plasma of individuals with the veryrare phenotype p, and anti-P is regularly found in P1k and P2k individuals Theseantibodies can be either IgM or IgG

Anti-Tja and anti-P are usually complement binding and hemolytic in vitro They can

give rise to severe blood transfusion complications involving intravascular

hemol-ysis if incompatible RBCs are transfused It is therefore essential to select neic blood of the correct phenotype, but the rarity of these phenotypes is such

alloge-that this will not always be possible Autologous blood transfusion will be the

method of choice whenever possible

There appears to be a significantly increased risk of spontaneous abortion in womenwho have the p phenotype, in particular if anti-Tja of the IgG3 subclass is present

Patients with paroxysmal nocturnal hemoglobinuria usually have auto anti-P that

acts as a biphasic hemolysin

A nuclear hypolobulation of granulocytes (best seen in neutrophils) The condition can

be inherited or acquired The inheritance is autosomally dominant (incidence 1:1000 to1:10,000), and typically bilobed neutrophils are seen, with occasional mononuclear forms(Figure 95) In the rare homozygous patient, all neutrophils are mononuclear Pelger-Huëtcells appear to be functionally normal, and increased infections are not seen

The acquired form (often called pseudo-Pelger-Huët cells) classically occurs in

myelo-dysplasia, but other conditions, e.g., myxedema, can also give rise to this appearance ThePelger-Huët cells of myelodysplasia are often hypogranular and functionally defective

PERIPHERAL-BLOOD-FILM EXAMINATION 697

PELIOSIS

The occurrence of cystic spaces in the spleen and liver filled with blood They are usually

an incidental finding in the parafollicular areas of lymphoid follicles, particularly in the

marginal zones They can cause hepatosplenomegaly and are associated with

thromb-ocytopenia.

PENTOSE PHOSPHATE PATHWAY

See Red blood cell — metabolism.

PERFORINS

Monomeric proteins present in the granules of NK lymphocytes and cytotoxic

T-lympho-cytes They are probably produced in the spleen They are activated by the cascade of

serine proteases, one of which is immunologically homologous with the ninth component

of complement, to form pores in the target cell membrane

Deficiency of perforins is an autosomally recessive disorder of dysregulated immune

response: familial hemophagocytic lymphohistiocytosis.262a The patients have a febrile

illness with hepatosplenomegaly, pancytopenia, hyperglyceridemia, CSF pleocytosis

(50%), and widespread neurological abnormalities The bone marrow is hypoplastic or

aplastic, with hemophagocytosis evident.408

PERIPHERAL-BLOOD-FILM EXAMINATION

Microscopic examination of a stained blood film This provides information for:

Diagnosis of blood disorders

Checking the blood count parameters obtained from automated instruments

Blood films are routinely prepared from EDTA specimens up to 3 h following ture Alternatively, they can be prepared using freshly collected capillary or nonanticoag-ulated venous blood Use of nonanticoagulated blood results in platelet clumping Thetraditional wedge smear remains the most popular It should be 2.5 to 3.0 cm long and2.0 to 2.5 cm wide and have a smooth surface free from holes and ridges with a shortfeather edge The film is stained by a Romanowsky method, the principal components ofwhich are Azure B and Eosin Y

venepunc-FIGURE 95

Diagrammatic representation of nuclear shapes in Pelger-Huët anomaly.

Red blood cells

1 Depth of staining (hemoglobin content)

• Codacyte (target cell)

• Dacrocyte (teardrop cell)

• Drepanocyte (sickle cell)

• Crenated cells (artifacts)

• Pyknocyte (irregular contracted cell, bite cell)

• Spiculated (Burr) cells: acanthocyte, echinocyte

• Fragmented cells: keratocyte (horn cell), schistocyte (helmet cell)

PERIPHERAL T-CELL LYMPHOMA, UNSPECIFIED 699

Granulocytes (neutrophils, eosinophils, and basophils)

6 Immature granuloctyes (myeloblasts, myelocytes, metamyelocytes)

Mononuclear cells (monocytes and lymphocytes)

1 “Reactive” lymphocytes — plasma cells, Mott cells

2 Atypical mononuclear cells

3 Immunoblasts (Turk cells)

4 Lymphoblasts, monoblasts, lymphoma cells

Platelets

1 Giant platelets — Bernard-Soulier syndrome

2 Granular depletion — Gray platelet syndrome

3 Platelet clumping

4 Platelet satellitism

PERIPHERAL-BLOOD STEM-CELL TRANSPLANTATION

(PBSC transplantation) See Autologous bone marrow transplantation.

PERIPHERAL T-CELL LYMPHOMA, UNSPECIFIED

(Rapaport: diffuse poorly differentiated lymphoma, diffuse mixed lymphocytic-histiocyticlymphoma; Lennert: lymphoepithelioid cell lymphoma; Lukes-Collins: T-immunoblastic

lymphoma) See also Lymphoproliferative disorders; Non-Hodgkin lymphoma.

A predominantly nodal heterogeneous group of aggressive T-cell lymphomas They arecharacterized by having widespread disordered lymphoid follicles showing a diffuse oroccasionally interfollicular cellular proliferation This ranges from atypical small cells tomedium-sized or large cells; most contain a mixed population of small and large atypicalcells, and even those with a predominance of medium-sized or large cells often contain abroad spectrum of cell sizes The neoplastic cells often have irregular nuclei and varyconsiderably in size and shape, with occasional large, hyperchromatic cells that may

resemble Reed-Sternberg (RS) cells, but true RS cells are rare or absent Admixed

eosino-phils or epithelioid histiocytes may be numerous There is a T-zone variant and a

lym-phoepithelioid cell variant The immunophenotype of the tumor cells shows variation in

700 PERNICIOUS ANEMIA

T-cell-associated antigens (CD3+/ −, CD2+/ −, CD5+/ −, CD7−/+) and B-cell-associated antigens.Most nodal cases are CD4+, CD8−, and CD30+ in the large-cell variants Cytogenetic analysis

usually shows rearrangement of TCR genes, with Ig genes being germline The cellular

origin is a peripheral T-cell in various stages of transformation

Clinical Features

These comparatively uncommon tumors comprise less than 15% of lymphomas in Europeand the U.S., but are more common in other parts of the world Patients are usually adultspresenting with nodal involvement but also with hepatosplenomegaly, bone marrow

infiltration, and other visceral disturbances Occasionally, eosinophilia or

hematophago-cytic syndromes are present The clinical course is usually aggressive and the prognosispoor, with overall survival rates of 30% at 5 years, even with treatment, as relapses aremore common than in B-cell lymphomas of similar histologic grades

Staging

See Lymphoproliferative disorders.

Treatment

See also Non-Hodgkin lymphoma.

The standard treatment strategy has been CHOP chemotherapy (see Cytotoxic agents) at initial presentation and peripheral stem cell transplantation (PSCT) for relapsed patients.

They are a common manifestation of purpura (see Hemorrhagic disorders).

PEUTZ-JEGHER’S SYNDROME

See Oral mucosa disorders.

PHAGOCYTOSIS

The internalization of particulate matter by cells into cytoplasmic vesicles It is a form of

endocytosis in which large particles (e.g., cell debris) are taken up and engulfed into a

cell The particle is progressively surrounded by cell pseudopodia in which actin-binding proteins accumulate Lysosomes fuse with the particle, so that the resulting endocytic vesicle is converted into a phagosome.

Phagocytes are neutrophils, monocytes, and histiocytes (macrophages) derived from

the same myeloid progenitor stem cell Activation of phagocytes initiates “respiratoryburst,” which kills phagocytosed organisms

PHLEBOTOMY 701

PHAGOSOMES

See Phagocytosis.

PHARYNGEAL DISORDERS

The changes in the pharynx associated with hematological disorders:

Pharyngitis with sore throat, dysphagia, tonsillar swelling, and ulceration with

Postcricoid carcinoma, complicating epithelial webbing

Lymphoproliferative disorder of the tonsils

Nasopharyngeal carcinoma associated with Epstein-Barr virus (EBV) infection;

adop-tive immunotherapy to EBV-specific T-cells has a potential therapeutic effect

PHENOTYPE

The observable characteristics of a cell or organism resulting from the interaction betweenits genetic components and the environment Most commonly used in hematology to

define cell-surface antigenic profile, such as the use of monoclonal antibodies in

immu-nophenotyping for the diagnosis of leukemias and antisera for the characterization of red

blood cell antigens (see Blood groups).

PHI BODIES

Polyribosomal spindle-shaped hydrogen peroxide-positive parent organelles in the

cyto-plasm of blast cells from patients with acute myeloid leukemia They are named after

their resemblance to the Greek letter f (φ) They are also present in leukemic promyelocytes

and in the blast cells of myelodysplasia (MDS).

compo-rubra vera and other forms of severe erythrocytosis, therapeutic phlebotomy is performed

to reduce the circulating red blood cell mass, to alleviate hyperviscosity, and to induceiron depletion The removal of one unit of blood (440 to 500 ml) once weekly is usuallyadequate In most patients with hemochromatosis or other forms of iron overload who

do not have severe anemia, a similar phlebotomy schedule is used Persons with severe

hyperviscosity or iron overload sometimes tolerate and benefit from phlebotomy twice

702 PHOSPHODIESTERASE INHIBITORS

weekly In women, the elderly, or persons of small body mass, the removal of one-halfunit of blood per session is sometimes better tolerated than larger volumes Patients withhyperviscosity or symptoms of hypovolemia after phlebotomy may benefit from theinfusion of crystalloid solutions of the approximate volume of the blood removed One

unit of blood in persons with a normal hemoglobin concentration contains approximately

200 mg iron, permitting estimation of the quantity of iron removed over a series of

phlebotomy treatments (“quantitative phlebotomy”)

PHOSPHODIESTERASE INHIBITORS

A group of substances that potentiate drugs acting on platelets and that are dependent

on cyclic nucleotide generation They act by reducing the breakdown of cyclic nucleotides,effectively increasing the concentration of cAMP, which favors the movement of Ca2+ into

the dense bodies, where it is metabolically inert Dipyrimadole inhibits adenosine

reuptake as well as inhibition of cAMP and cGMP phosphodiesterase, leading to

main-tained levels of intraplatelet cAMP This potentiates the action of prostacyclin (PGI2).

Theophylline, a specific cAMP phosphodiesterase inhibitor, also has antiplatelet effects.Dipyrimadole is also a potent vasodilator and may lead to faintness/headaches Its actions

are synergistic with those of aspirin.

PHOSPHOFRUCTOKINASE

See Phosphohexose kinase.

6-PHOSPHOGLUCONATE DEHYDROGENASE

An enzyme of the hexose monophosphate shunt of red blood cell metabolism, deficiency

of which is rare and only gives rise to hemolysis if the patient is taking primaquine

PHOSPHOGLUCONATE PATHWAY

(Pentose phosphate pathway) See Red blood cell — metabolism.

PHOSPHOGLYCERATE KINASE

An enzyme of the Embden-Meyerhof pathway of red blood cell metabolism responsible

for the conversion of 1,3-diphosphoglycerate (1,3-DPG) to 3-phosphoglycerate, thus

gen-erating adenosine triphosphate (ATP) from adenosine diphosphate (ADP) It is the only

enzyme of the anaerobic glycolytic pathway known to be encoded on the X chromosome.Deficiency of this enzyme causes the level of 2,3-diphosphoglycerate (2,3-DPG) to rise andthat of ATP to fall, which reduces the Na+K+ pump of its preferred source of energy, causing

Na+ accumulation in the cell, with eventual lysis It is a rare enzyme deficiency, but thosecases studied have shown considerable polymorphism Roughly one-half of affected males

show hemolytic anemia of variable severity accompanied by neurological and mental

abnormalities Females show mosaicism and may have mild hemolysis If severe hemolysis

exists in phosphoglycerate kinase deficiency, the anemia is ameliorated by splenectomy.

Neurologic complications, however, rather than hemolysis, tend to dominate the clinicalfeatures

PICKWICKIAN SYNDROME 703

PHOSPHOGLYCEROMUTASE

An enzyme of the Embden-Meyerhof pathway of red blood cell metabolism, which

reversibly catalyzes the reaction 3-phosphoglycerate to 2-phosphoglycerate Deficiency ofthis enzyme has not been reported

PHOSPHOHEXOSE ISOMERASE

An enzyme of the Embden-Meyerhof pathway of red blood cell metabolism that catalyzes

the conversion of glucose 6-phosphate to fructose 6-phosphate Deficiency causes a

hemolytic anemia that responds to splenectomy Progressive neurological degeneration

See Glyceraldehyde-3-phosphate dehydrogenase.

PHYSICALLY INDUCED DISORDERS

The hematological disorders induced by physical agents:

Traumatic disorders causing hemorrhage and disseminated intravascular

coagu-lation

Mechanical damage to red blood cells: macrovascular hemolytic anemia, cardiac hemolytic anemia, march hemoglobinuria

Burns hemolytic anemia

Ultraviolet light as a possible etiological factor of lymphoproliferative disorders

Ionizing radiation affecting bone marrow

Heat stroke causing neutrophil botyroid nuclei

hyper-in a return to normal ventilation, and some patients may benefit from sublhyper-ingual progesterone

medroxy-704 PINCER RED BLOOD CELLS

PINCER RED BLOOD CELLS

Red blood cells having the shape of pincers, as seen in peripheral-blood smears Themechanism leading to this morphological abnormality is a partial deficiency of band 3protein in the red cell membrane This is an autosomally dominant hereditary defect Due

to the instability of this band 3 protein, the morphology becomes more obvious in aging

cells Its clinical behavior, consequences, and treatment are similar to those of hereditary

spherocytosis, and it is recognized as one of the red cell membrane-linked protein defectsleading to this disorder

PINOCYTOSIS

The uptake of soluble substances by granulocytes The mechanism is analogous to

phago-cytosis, in which particulate matter is taken up, the processes being virtually identical,i.e., projections from the cell membrane surround the substance and form a vesicle that

is then internalized The process is not visible by light microscopy Pinocytosis occurs via

specific membrane-bound receptors, e.g., light-density lipoprotein, granulocyte colony

stimulating factor (G-CSF), granulocyte/macrophage colony stimulating factor

(GM-CSF), etc., or may be receptor independent

PITUITARY GLAND DISORDERS

The interrelationship of hematological and pituitary gland disorders The neuropeptide

axis provides direct communication The hormones produced by the pituitary gland inthis interaction include prolactin, macrophage-inhibiting factor, and substance P These

hormones antagonize corticosteroids and stimulate T and B-lymphocytes and histiocytes

(macrophages) Any immune disorder may therefore induce a change in the pituitary-adrenal axis Specific pituitary disorders include:

hypothalamic-Hypopituitarism as a consequence of:

• Hemorrhage or infarction (pituitary apoplexy)

• Infiltration by sarcoidosis or histiocytosis

• Lymphocytic hypophysitis in a general autoimmune disorder

Anterior pituitary adenoma variable effects:

• Normocytic anemia as a result of thyroid and androgen deficiency

• Erythrocytosis through the effect of growth hormone on renal release of

The fluid in which the blood cells are suspended It contains many proteins, some of which

are enzymes, e.g., coagulation factors and erythropoietin as well as electrolytes and

carbohydrates

PLASMACYTIC LYMPHOCYTIC LYMPHOMA 705

PLASMABLAST

Undifferentiated plasma cells that have a central large nucleus containing several

prom-inent nucleoli and relatively scant cytoplasm Some may be binucleate or multinucleate

They are a characteristic feature of plasma cell neoplasms (myelomatosis) and

plasmacy-tomas, where an increase in plasmablasts may confer a worse prognosis

PLASMA CELL

See also Lymphocytes — B-cells; Immunoglobulins.

An end-stage B-lymphocyte with an eccentric round nucleus with “clock-face” chromatin

pattern The cytoplasm is strongly basophilic apart from a perinuclear light-staining Golgibody They are rarely seen in normal peripheral blood but are prominent in chronicinflammatory disorders Large numbers occur in the peripheral blood and bone marrow

in association with plasma cell neoplasms.

PLASMA CELL MYELOMA

See Myelomatosis.

PLASMA CELL NEOPLASMS

Immunosecretory disorders resulting from the expansion of a single clone of

immunoglo-bulin-secreting, terminally differentiated end-stage B-lymphocytes These monoclonal

proliferations of either plasma cells or plasmacytoid lymphocytes are characterized by

secretion of a single homogeneous immunoglobulin: M-protein (see Monoclonal

gamm-opathies) The neoplasms are classified as:

Plasma cell myelomas (myelomatosis)

Monoclonal light and heavy chain deposition diseases

Osteosclerotic myeloma (POEMS syndrome)

Heavy-chain diseases (HCD)

PLASMACYTIC LYMPHOCYTIC LYMPHOMA

See Lymphoplasmacytic lymphoma/Waldenström macroglobulinemia, Non-Hodgkin

lymphoma

706 PLASMACYTOID DENDRITIC CELL

PLASMACYTOID DENDRITIC CELL

See Dendritic reticulum cells.

PLASMACYTOMAS

Clonal proliferations of plasma cells that are cytologically and immunophenotypically identical to those of plasma cell myeloma (myelomatosis, multiple myeloma), but manifest

a localized osseous or extraosseous growth pattern.410

Solitary Plasmacytoma of Bone

The diagnosis depends on histologic evidence of a plasma cell tumor In addition, completeskeletal radiographs must show no other lesions; the bone marrow aspirate must contain

no evidence of multiple myeloma; and immunoelectrophoresis or immunofixation of theserum and concentrated urine should show no M-protein Exceptions to the last criterionoccur, but therapy of the solitary lesion usually results in disappearance of the M-protein.Treatment consists of radiation in the range of 40 to 50 Gy Overt multiple myelomadevelops in approximately 55% of patients during 10 years of follow-up New bone lesions

or local recurrence develops in 10% of cases There is no evidence that adjuvant therapy influences the incidence of conversion to multiple myeloma

chemo-Extramedullary Plasmacytoma

This is a plasma cell tumor that arises outside the bone marrow It is located in the upperrespiratory tract in approximately 80% of cases However, solitary extramedullary plas-macytomas can occur in virtually any organ They usually spread locally, but multiplemyeloma may develop There is a predominance of IgA M-proteins The diagnosis is based

on the finding of a plasma cell tumor in an extramedullary location and the absence ofmultiple myeloma upon bone marrow examination, radiography, and appropriate studies

of serum and urine Treatment consists of tumoricidal radiotherapy Regional recurrence

occurs in approximately one-fourth of patients, but development of typical multiplemyeloma is uncommon

PLASMA EXCHANGE

See Hemapheresis.

PLASMAPHERESIS

See Hemapheresis.

PLASMA THROMBOPLASTIN ANTECEDENT

See Factor XI.

PLASMA THROMBOPLASTIN COMPONENT

See Factor IX.

PLATELET 707

PLASMA VOLUME

See also Blood volume.

The quantity of liquid plasma in the circulation

PLASMIDS

Circular DNA that occurs naturally in microorganisms and is used for joining DNA

fragments in large quantities

Nonnucleated cytoplasmic fragments derived from bone marrow megakaryocytes They

are smooth, biconvex disks, 1 to 4 µm in diameter, with a role in primary hemostasis.Adequate numbers are therefore required for normal hemostasis

Once released from the marrow, platelets are sequestered in the spleen for 24 to 48 h.The spleen can contain up to 30% of the normal circulating mass of platelets This pro-

portion can be significantly increased in splenomegaly, leading to thrombocytopenia It

is not known if these represent newly formed cells Significant platelet pools may alsoexist in the lungs

The reference range for platelets in peripheral blood is 140 to 400 × 109/l Plateletcounting shows variations in individuals with exercise, stress, and menstrual cycle Racialdifferences are observed, with lower reference ranges seen in some Mediterranean races

(see Mediterranean thrombocytopenia).

Normal platelet life span is 8 to 14 days, dependent upon the method by which plateletsurvival time is measured Platelets are removed from the circulation by the reticuloen-dothelial system on the basis of senescence rather than by random utilization However,there is a small fixed component to platelet turnover due to random utilization of plateletsthat maintain vascular integrity

Platelet Structure

The main structural features are shown diagrammatically in Figure 96 The cell membranelipid bilayer is partially or completely penetrated by a range of glycoprotein molecules.These function as receptors for a range of different agonists, adhesive proteins, coagulationfactors, and other platelets Specific membrane glycoproteins have been characterized withtheir associated functions (see Table 127)

The most abundant glycoproteins on the platelet surface are glycoprotein IIb/IIIa (GpIIb/IIIa) These two glycoproteins form a heterodimer and carry receptors for fibrin-

ogen , Von Willebrand Factor (VWF), and fibronectin (adhesive proteins) The GpIIb/IIIa complex is a member of the integrin family of cellular adhesion molecules Glycoprotein

Ib is also important, as this contains a receptor for Von Willebrand Factor and thrombin.This receptor plays an essential part in the platelet–vessel wall interaction A less well-

defined group of 7-transmembrane domain glycoproteins can be released in adenosine

708 PLATELET

deaminase (ADA) adrenaline- and thrombin-mediated aggregation Deficiencies of

plate-let membrane glycoproteins lead to disorders of plateplate-let function In addition, membraneglycoproteins act as specific alloantigenic sites The cell membrane also contains phospho-

lipid, associated with prostaglandin synthesis, calcium mobilization, and localization of

coagulant activity to the platelet surface

Below the cell membrane lies the peripheral band of microtubules, which functions asthe cell cytoskeleton The microtubules maintain the discoid shape in the resting plateletbut disassemble upon platelet aggregation and then reappear toward the center of thecell, entrapping granules This is thought to help the release reaction

The surface-connected canalicular system is an extensive system of plasma membraneinvaginations It increases the surface area across which membrane transport can occurand through which platelet granules can discharge their contents during the secretoryphase of platelet aggregation Dilated canaliculi are the probable explanation for vacuolesseen in normal platelets

The dense tubular system is smooth endoplasmic reticulum, which is the site of

pros-taglandin synthesis and sequestration/release of calcium ions

FIGURE 96

Diagrammatic representation of platelet ultrastructure.

TABLE 127

Important Platelet Membrane Glycoproteins

Glycoprotein 10 3 Copies per Platelet Receptors

PLATELET 709

Platelets also contain many organelles, including mitochondria, glycogen granules,

lysosomes, peroxisomes, and two types of platelet-specific granules Platelet-specific ules are either dense osmophilic granules (dense bodies, δ-granules) or α-granules Densebodies contain 60% of the platelet storage pool of adenine nucleotides (such as adenosinediphosphate) and serotonin Dense-body adenine nucleotides do not readily exchangewith other adenine nucleotides in the platelet metabolic pool The α-granules contain aseries of different proteins, some of which are platelet specific and some of which arefound in the plasma or other cell types, such as coagulation factors Major contents of α-granules include VWF, platelet factor 4, β-thromboglobulin, thrombospondin, factor V,

gran-fibrinogen, fibronectin, platelet-derived growth factor, high-molecular-weight kininogen,and tissue plasminogen activator inhibitor-1 Contents of the platelet-specific granules aresecreted in response to aggregating stimuli

Platelets have a number of specific antigens on their surface, many associated withplatelet membrane glycoproteins Ia, Ib, Iib, IIIa, and possibly IV and V, such as HPA-1Aassociated with glycoprotein IIIa These may be shared with other cells that have theadhesion receptors GP Ia and IIIa, which include vascular endothelium cells, smooth-muscle cells, fibroblasts, and activated T-cells There are no naturally occurring antibodies,these only arising from reaction to transfused platelets or placental transfer

Platelets also express human leukocyte antigen (HLA) class I antigens and ABO blood

group antigens These are of importance in immunological refractoriness to platelet

trans-fusions

Platelet Function

See also Hemostasis.

In the presence of vessel wall injury, escaping platelets come into contact with and adhere

to collagen and subendothelial bound Von Willebrand Factor (VWF), through

glycopro-tein Ib Glycoproglycopro-tein IIb/IIIa is then exposed, via VWF binding, and forms a secondbinding site for VWF In addition, with exposure of the GpIIb/IIIa site, fibrinogen may

be bound to promote platelet aggregation Within seconds of adhesion to the vessel wall,platelets begin to undergo a shape change, possibly due to ADP released from the damagedcells or other platelets exposed to the subendothelium Platelets become more sphericaland put out pseudopods, which enables platelet-platelet interaction The peripheral micro-tubules become centrally apposed, forcing the granules toward the surface and the surface-connected canalicular system Platelets then undergo a specific release reaction of theirgranules The intensity of the release reaction is dependent upon the intensity of thestimulus With the shape change, there is also further exposure of the GpIIb/IIIa complex,with further binding of fibrinogen As fibrinogen is a dimer, it can form a direct bridge

between platelets or act as a substrate for the lectinlike protein thrombospondin With

the enhancement of platelet-platelet interaction, platelet aggregation ensues Plateletaggregation causes activation, secretion, and release from other platelets, thus leading to

a self-sustaining cycle that results in the formation of a platelet plug.

The binding of agonist to platelet receptors not only leads to expression of fibrinogen

receptors (glycoprotein IIb/IIIa), but also a series of cell signal transduction events that

mediate the release reaction (see Figure 97)

Agonist receptor interaction leads to activation of guanine nucleotide-binding proteins(G protein) and hydrolysis of plasma membrane phospholipids (phosphotidyl inositides)

by phospholipase C (PLC) The inositol triphosphates formed act as ionophores andmobilize calcium ions in the cytosol (cytosol calcium) from the dense tubular system, andlead also to an influx of calcium from outside of the cell Diacylglycerol, also formed withinthe G protein/PLC pathway, activates protein kinase C, which in turn phosphorylates a

710 PLATELET

47-kDa contractile protein Together with the calcium-dependent phosphorylation of sin light chain, these reactions induce contraction and secretion of granule contents CyclicAMP/adenyl cyclase exerts regulatory control over these reactions (high levels of cAMPreduce cytosol calcium concentration) and are in turn regulated by G protein activity Inaddition, prostaglandin (cyclic endoperoxides and thromboxane A2) synthesized frommembrane phospholipids may bind to specific receptors and further stimulate these pro-cesses

myo-Among the content of platelet α-granules are several coagulation factors, such as factor

V, fibrinogen, and high-molecular-weight kininogen Upon secretion from the α-granule,these factors reach high local concentrations Platelets function to provide a local phos-pholipid surface for these factors to work upon, particularly factor V This procoagulantactivity of platelets is not seen in resting platelets It is due to release of granule contentsand redistribution of platelet membrane phospholipids upon secretion and activation.Platelet function is studied by the use of platelet aggregating agents whose effects aremeasured by a platelet aggregometer

Platelet Disorders

These are due to either platelet deficiency (thrombocytopenia) or platelet-function

dis-orders Their treatment depends entirely upon the cause

Platelet Antagonists

Inhibition of platelet aggregation can be induced by the action of drugs These antiplatelet

drugs are widely used for prevention of thrombosis (see Arterial thrombosis; Venous

thromboembolic disease ) Aspirin, ticlopidine, and clopidogrel are effective for long-term

prophylaxis.82,86

FIGURE 97

Cell signal transduction in the platelet-release reaction PIP2, phosphotidylinositol; IP3 and IP4, inositol tri- and tetraphosphates; PLC, phospholipase C; PKC, phosphokinase C; DG, diacylglycerol; MLCK, myosin light-chain kinase.

Platelet counts are used for:

Screening for blood disorders

Transfusion management in thrombocytopenia

Monitoring cytotoxic therapy or recovery from allogeneic or autologous stem cell

transplantation

The clinical value of platelet counts is at its greatest between 10 and 50 × 109/l; doxically, it is within this range where the accuracy of all methods is most doubtful Atlow concentrations, different counts can be produced in the impedance and optical chan-nels of the same automated instrument due to different interferences These differencesare algorithmically exploited in some instruments to produce a “correct” count

para-The physiological count range is 150 to 400 × 109/l, showing minor diurnal variationand day-to-day variation Females have counts about 20% higher than males, falling withmenstruation Around 2% of those living around the Mediterranean have counts less than

90 × 109/l (see Mediterranean thrombocytopenia) There are no differences with age,

apart from infants having levels at the lower end of the normal range Strenuous exercise

raises the level by 30 to 40% The values for infants and children are given in Reference

Range Table X

PLATELET FACTOR 4

A cationic polypeptide synthesized by megakaryocytes In platelets, it exists as

polypep-tide α-granules that inhibit collagenases of neutrophils and fibroblasts.

712 PLATELET-FUNCTION DISORDERS

Inherited Disorders

Disorders of Platelet Membranes

These may be due to deficiency of membrane glycoproteins that are important for platelet

aggregation Bernard-Soulier syndrome is due to deficiency of glycoprotein Ib/IX

com-plex on the platelet surface Platelets deficient in glycoprotein Ib are unable to interact

Rare membrane protein deficiency Deficiency of platelet procoagulant activity Disorders of platelet secretion

Defects of secretory mechanism Weak agonist response defects

Cyclooxygenase deficiency

Thromboxane synthetase deficiency

Deficiency of platelet granules Storage pool disorders

α-Granule deficiency (Gray-platelet syndrome)

δ -Granule deficiency Combined α - and δ -granule deficiency

Acquired Disorders

Hematological disorders

Dysproteinemic purpura Leukemias

Myelodysplasia Myeloproliferative disorders

Systemic disorders

Autoimmune disorders Burns (severe)

Transplant rejection Drugs

Alcohol Aspirin

Cephalosporins

Dextran Dipyrimadole Heparin

Nonspecific anti-inflammatory drugs (NSAIDS) Penicillins

Selective serotonin-uptake inhibitors Radiographic contrast agents Sulfinpyrazones

Prostaglandin overproduction

con-Deficiency of Platelet Procoagulant Activity

(Scott’s syndrome) This is a rare bleeding disorder The defining characteristic is the

absence of Ca2+-stimulated exposure of phosphatidylserine (PS) from the inner leaflet ofthe plasma membrane bilayer to the cell surface This normally provides a surface for theassembly of the “Xase” and “prothrombinase” complexes of the coagulation Consequentlythe “Xase” and “prothrombinase” complexes cannot assemble on the platelet membrane

The adenosine triphosphate (ATP)-binding cassette transporter A1 (ABCA1) has been implicated as a potential cause Bleeding problems are corrected by platelet transfusion

of normal platelets

In these rare cases of bleeding, it is proposed that they are due to insufficient binding

sites for factor X and factor V Consequently the “tenase” and “prothrombinase”

com-plexes cannot assemble on the platelet membrane Bleeding problems are again corrected

by platelet transfusion of normal platelets

Disorders of Platelet Secretory Mechanism

These arise from abnormalities of either platelet secretory mechanisms or from deficiencies

of one or more types of platelet granules These disorders lead to a mild-to-moderate

bleeding tendency, characterized by features of platelet-based hemorrhage, bruising,

epistaxis, menorrhagia, postoperative bleeding, and bleeding after dental extraction.Disorders due to failure of platelet secretory mechanisms constitute a heterogeneousgroup In most cases, the causes of the failure of the secretory/release mechanism are

unknown Exceptions include a few patients who have definable cyclooxygenase or

thromboxane A2 synthetase deficiency Platelet-aggregation studies show impairment of

aggregation to weak agonists such as ADP, adrenaline/epinephrine, or low concentration

of collagen Characteristic impairment is seen as first-wave aggregation, but with absence

of second wave Higher concentrations of agonists may lead to second-wave aggregation,

albeit slow Failure to aggregate in response to arachidonic acid implies a defect in the

prostaglandin pathways Defects of secretion without a clear cause can be grouped asweak agonist response defects, so-called WARD syndrome Bleeding in patients with

secretory defects can be managed by transfusion of normal platelets DDAVP is often used

as a first-line treatment, as it improves hemostatic function in patients with such function disorders The mechanism of this improvement is unknown DDAVP may besufficient for surgical prophylaxis if patients are known to respond Local control of

platelet-bleeding and concomitant use of antifibrinolytic agents such as tranexamic acid are

impor-tant adjunctive measures

Storage-Pool Disorders of Platelets Acquired Disorders412

Myeloproliferative disorders and myelodysplasia may be associated with defective platelet function, in addition to the observed thrombocytosis or thrombocytope-

nia No one particular mechanism is responsible for the observed defects of

function Impairment of platelet function is frequently seen in dysproteinemic

purpura, including impaired platelet aggregation and adhesion, which correlatedwell with bleeding tendency

714 PLATELET-FUNCTION DISORDERS

Renal disorders are a very common cause of an acquired platelet-function defect ing in renal disease is multifactorial, although platelet dysfunction is a major com-ponent Platelets in renal disease show a diverse range of biochemical defects.Function is impaired by urea, guanidinosuccinic acid, and other phenolic metabolitesknown to accumulate in renal failure Dialysis corrects, in part, platelet-functiondefects by removal of interfering compounds Transfused normal platelets willacquire the same defect as native platelets after a few hours in the circulation

Bleed-Liver disorders often cause a platelet-function defect The mechanisms involved areunknown and are probably multifactorial

Systemic disorders include autoimmune disorders, immune thrombocytopenic

pur-pura (ITP), thrombotic thrombocytopenic purpura (TTP), hemolytic uremic

syn-drome (HUS), disseminated intravascular coagulation (DIC), cardiopulmonary

bypass, transplant rejection, burns, and valvular heart disease Such diverse ditions can induce an acquired storage pool defect This can be mediated bydamage to circulating platelets, either mechanical or immune, which leads topartial release of platelet granule contents Continued circulation of such depletedplatelets can lead to bleeding through a partial storage-pool-like defect Chronichypoglycemia can produce a similar defect by failure of normal-platelet glucose-based metabolism

con-Adverse drug reactions174,412 (see Table 128) comprise the commonest causes of anacquired platelet-function disorder Of the drugs that impair platelet function,

aspirin is by far the most important Aspirin inhibits cyclooxygenase and results

in platelets failing to synthesize prostaglandin endoperoxides and thromboxaneA2 The acetylation of cyclooxygenase by aspirin is irreversible Other non-specific anti-inflammatory drugs (NSAIDs) have similar effects but a shortereffect Most NSAIDs only inhibit platelets for the duration while they or theirmetabolites are in the circulation Aspirin and NSAIDs produce a profoundplatelet release defect/failure of the secretory mechanism Platelet aggregationstudies show only first-wave aggregation with ADP, adrenaline/epinephrine,and collagen Arachidonic acid-induced aggregation is classically absent Theantiplatelet effects of these drugs can readily be treated by their discontinuance

Phosphodiesterase inhibitors such as dipyrimadole are sometimes used phylactically, being synergistic to aspirin Prostacyclin can also be used for itssynergistic effect with heparin for cardiopulmonary bypass procedures and tosupport renal dialysis b-Lactam antibiotics (e.g., penicillins, cephalosporins) areknown to impair platelet function The effects are seen only in patients receivinglarge doses of parenteral antibiotics The mechanism of action is unclear, butmay be due to the adsorption of the antibiotic onto the platelet membrane, whichthen blocks multiple receptor-agonist interactions In addition, they may alsohave an effect upon calcium influx in response to platelet stimulation A number

pro-of foods are known to have effects upon platelets and platelet function Forexample, garlic is known to inhibit both fibrinogen binding to platelets andplatelet aggregation, while ethanol has been shown to decrease platelet interac-tion with endothelial cells Diets deficient in arachidonic acid but rich in omega-

3 fatty acids lead to a reduction in the synthesis of thromboxane A2 (TxA2) andthe synthesis of TxA3 — the latter having no platelet aggregatory effect Incontrast, within the vessel wall, synthesis is reduced and the synthesis of PGI3

is increased; the latter is a potent antiaggregating agent

PLATELET-FUNCTION TESTING 715

Prostaglandin overproduction (Bartter’s syndrome) is a rare metabolic disorder thatmay cause an acquired platelet-function disorder It can be corrected by a highsodium intake

Acquired platelet disorders vary in the degree to which they might lead to clinically

significant bleeding per se Principles of treatment include correcting or treating the

under-lying disorder as far as possible Adjunctive treatments may include normal platelettransfusion or use of DDAVP

PLATELET-FUNCTION TESTING

The range of tests available for the estimation of the activity of a subject’s platelets.413 Acareful drug history must always be taken prior to platelet-function testing to ensure thatdrugs that affect platelet function have not been ingested Ideally, none of these drugsshould have been ingested in the preceding 2 weeks

A global estimation of platelet function can be made using the bleeding time test.

Reproducible results require the use of a template methodology Prolongation of bleedingtime in the face of a normal platelet count is indicative of a platelet-function disorder.Methods of assessing platelet adhesion are time consuming and unsuitable for clinical

application Platelet aggregation by a range of agents that cause platelets to aggregate in vitro is widely used to measure and assess platelet function using an aggregometer.

Platelet Aggregometers

These instruments are based on the principle that the absorbance of platelet-rich plasma falls

as platelets aggregate The amount and rate of fall are dependent on platelet reactivity to theagonist added if other variables (temperature, mixing speed, and platelet count) are constant.Changes in light absorbance (or transmission) are recorded on a chart recorder, and percent-age platelet aggregation to each agonist is determined using platelet-poor plasma as control.Platelet aggregometry is subject to a number of technical variables, including preparation ofplatelet-rich plasma, time since preparation, pH, and system optics

Platelet-Aggregating Agents

These include adenosine diphosphate (ADP), adrenaline/epinephrine, serotonin,

throm-bin , collagen, arachidonic acid, prostaglandins, thromboxane, and ristocetin All, with

the exception of ristocetin, are present in the circulation or endothelium/subendothelium

In vitro behavior in response to these agonists is not reflected by in vivo behavior in all

cases This is due to the artificial nature of the platelet aggregometer and the multiple

factors that determine in vitro platelet aggregation Of these agents, five (ADP, ristocetin,

collagen, arachidonic acid, and adrenaline/epinephrine) are commonly used in function tests

platelet-ADP is used at low concentrations to cause primary reversible “first phase” aggregation.Platelets may disaggregate after the first phase in the absence of continuing stimulus or

as a consequence of functional defect Higher concentrations of ADP cause irreversible

“second phase” aggregation, associated with the release of dense δ- and α-granules, andthe use of ADP alone may mask subtle functional defects

Adrenaline/epinephrine response is similar to that seen with ADP Some normal subjectshave impaired responses to low-dose adrenaline/epinephrine

Collagen aggregation response is preceded by a lag phase of 10 to 60 sec The lag phase

is inversely related to the concentration of collagen used Platelets then move into a single

716 PLATELET INDICES

wave of aggregation due to granule release and activation of arachidonic acid pathways.High-dose collagen leads to aggregation by release reactions alone and bypasses theprostaglandin-mediated pathways Both high and low collagen responses should be deter-mined

Arachidonic acid induces thromboxane generation and granule release Absence of aresponse to arachidonic acid is indicative of a defect in the arachidonic acid metabolicpathways

Ristocetin reacts with Von Willebrand Factor (VWF) to modulate the Von Willebrand

platelet glycoprotein Ib reaction It does not activate any other mechanism of plateletaggregation or granule release Poor responses are seen in patients with low levels of VWF

Increased response to low-dose ristocetin is indicative of either type 2B Von Willebrand

Disease (VWD) or pseudo-VWD

Different agonists have different patterns of aggregation The rate and extent of theaggregation of the sample are determined by the aggregometer Characteristic aggregationpatterns are seen in different disorders (see Figure 98)

Defects seen in response to weak agonists should always be repeated for confirmation.Platelets may sometimes aggregate in the absence of an agonist, so-called spontaneousplatelet aggregation This phenomenon is associated with arterial occlusive events

Reference Center Testing

Nucleotide content of platelets This determination is particularly important for the

diagnosis of storage pool disorders

Platelet release reaction This can be studied by use of bioluminescence techniques.

PLATELET INDICES

See also Automated blood cell counting.

FIGURE 98

Examples of platelet aggregation tracings Individual tracings run from bottom left (0% aggregation at time 0)

to top right (maximum aggregation seen after exposure to agonist).

PLATELET SATELLITISM 717

Calculations of platelet properties derived from platelet measurements

Plateletcrit Product of the platelet count and the mean platelet volume (MPV) — a

measurement produced by automated blood cell counters It has no clinical utility

Mean platelet volume (MPV) The platelet equivalent of the mean cell volume (MCV)

produced by multiparameter blood cell counters The instability of platelet volume

in ethylenediamine tetraacetic acid (EDTA) precludes its use With

aperture-impedance systems, there is an increase in MPV with increasing time from venoussampling Conversely, with light-scattering instruments, the MPV decreases uponstorage

Platelet distribution width (PDW) The platelet equivalent to the red blood cell

distri-bution width (RDW) The reference range in health is instrument specific Theonly clinical utility described for the PDW is in differentiating between reactive

thrombocytosis (normal PDW) and thrombocytosis associated with chronic

myeloproliferative disorders (increased PDW)

Reticulated platelet (immature platelet fraction, IPF) Newly released platelets containing

RNA are the platelet analog of the red blood cell reticulocyte A new automated

method to reliably quantitate reticulated platelets, expressed as the immature plateletfraction (IPF), has been developed The clinical utility of the IPF has been established

in the differential diagnosis between thrombocytopenia due to bone marrow failure and increased peripheral consumption, particularly immune thrombocytopenia

purpura (ITP) and thrombotic thrombocytopenic purpura (TTP).

Mean platelet component (MPC) An experimental parameter generated by a single

commercial instrument that measures platelet granularity using a laser source andlight-scattering technology as a putative index of platelet activation

PLATELETPHERESIS

See Hemapheresis.

PLATELET PLUG

The mass of aggregated platelet material formed after initial damage to a blood vessel as

a primary response in hemostasis It is an aggregate of platelets that have adhered at the

site of vessel injury It is soft and friable but functions to slow blood loss and allowsecondary hemostasis/fibrin clot to develop and seal the vascular defect

The presence of platelet rosettes around neutrophils It is a cause of an artifactually low

platelet count, particularly on automated counters Examination of the blood film will

718 PLATELET SURVIVAL MEASUREMENT

clearly demonstrate platelet rosetting as the cause of thrombocytopenia It is of no clinical

significance

PLATELET SURVIVAL MEASUREMENT

Determination of the time spent by platelets in the circulation.25 It is an important test

that helps to define the cause of thrombocytopenia, but the procedure is time consuming.

Older methods, in patients with decreased platelet production, include measuring thedecline in platelet count following transfusion of fresh platelets Commonly used methods

of measuring platelet survival nowadays involve radiolabeling platelets This can be doneusing either 51chromium, 111indium, or 113mindium compounds The 111In compounds areless radiotoxic than 51Cr compounds and are preferred for quantitative scintillation camera

imaging (See Radionuclides.)

Following injection of radiolabeled platelets, there is a rapid initial drop in the first 10

to 20 min, followed by a much slower fall This initial rapid decline is due to a combination

of splenic pooling and removal of platelets that have been damaged in the radiolabelingprocess The survival time is taken as the time after injection at which survival falls tozero Estimates of survival using 111In-labeled platelets are 9 to 10 days Platelet survivalcurves obtained in such studies fit a linear curve, implying that aging determines plateletlife span It is estimated that approximately 30% of platelets are pooled in the spleen,exchanging freely with circulating platelets Using 111In-labeled platelets, liver and spleenappear to be responsible for more than 80% of the clearance of senescent platelets, viareticuloendothelial phagocytosis Approximately 50% of platelets are removed from thecirculation by the spleen, 35% by the liver, and the remainder by the bone marrow, lymphnodes, and other tissues Following splenectomy, 75% are removed from the circulation

by the liver

Platelet production can increase two- to eightfold from a baseline of about 40 × 109

platelets/l/day In situations where platelet survival time is decreased, thrombocytopeniawill only ensue when the survival time is so short that it outstrips increased plateletproduction

PLATELET TRANSFUSION

The transfusion of platelet components of donated blood, the indications for platelettransfusion, and considerations concerning platelet transfusion refractoriness.414

Platelets are prepared by the differential centrifugation of units of whole blood or by

apheresis (see Blood components for transfusion — preparation and storage;

Hemapher-esis) Units of random donor platelets derived from single units of whole blood aregenerally pooled prior to administration Apheresis platelets are usually called units ofsingle-donor platelets It is now usual to prepare and issue platelets as an adult therapeuticdose This contains on average 3.0 × 1011 platelets in 200 ml of plasma For pediatric use,adult therapeutic doses can be split into smaller volumes or a single unit of random donorplatelets transfused

Indications for Transfusion

General guidelines

A decision to transfuse platelets is based on whether or not there is active clinical bleeding,deciding whether the patient is responsive to transfusions from unselected donors, and

PLATELET TRANSFUSION 719

monitoring the platelet count (see Table 129) At a platelet count of 10 to 100 × 109/l, thebleeding time shows a linear inverse correlation with the platelet count, suggesting thatthe count itself is a reliable indicator of platelet-mediated hemostasis At platelet counts

of <10 × 109/l, the bleeding time is >30 min

Platelet transfusion is not normally helpful in immune thrombocytopenic purpura (ITP) and is contraindicated in patients with thrombotic thrombocytopenic purpura (TTP).

Platelet-Function Disorders

Congenital Disorders

These involve defective membrane receptors and granules, as well as abnormal

biochem-ical pathways, and include Glanzmann’s thrombasthenia, Wiskott-Aldrich syndrome,

Bernard-Soulier syndrome , May-Hegglin anomaly, and Chediak-Higashi syndrome.

Acquired Disorders

These include drugs, commonly aspirin, that inhibit cyclooxygenase Patients with uremia,

myeloproliferative disorders , and myelodysplasia often have platelet dysfunction

Car-diopulmonary bypass procedures cause platelet dysfunction, since platelets become vated and then nonfunctional in the extracorporeal circuit

acti-Transfusion is indicated in patients who are known to have disordered platelet function,who undergo elective surgical procedures, and in cases where there is active clinicalbleeding Because such patients are often chronically transfused, it may be important to

prevent human leukocyte antigen (HLA) alloimmunization and subsequent transfusion refractoriness (see Blood components for transfusion — complications).

Platelet Refractoriness to Transfusion

After transfusion, the platelet count ideally should be >50 × 109/l Failure to increment

by >10 × 109/l at 1 to 20 h posttransfusion on two occasions is a simple practical indicator

of refractoriness, which should be investigated The likely causes include:

Nonimmune refractoriness: a much more common disorder than immune-mediated

refractoriness Accelerated platelet consumption is found in patients who havehad a stem cell transplantation (SCT), disseminated intravascular coagulation

(DIC), antibiotic or amphotericin-B therapy, splenomegaly, or hemorrhage.

Immune-mediated refractoriness: caused by the presence of antibodies The most

impor-tant are human leukocyte antigen (HLA) antibodies; others are platelet-specificalloantibodies, platelet autoantibodies, ABO antibodies, and drug-induced plateletantibodies

TABLE 129

Guidelines for Transfusion of Platelets

Count

(10 9 /l) Decision

<10 Transfusions should be given to stable patients without any evidence of bleeding.

<20 Transfusions indicated where there is additional risk factors for bleeding, including fever, sepsis, and

coexistent coagulopathy.

<50 Transfusions indicated where invasive procedures are planned or there is ongoing hemorrhage.

<100 Transfusions should be given if invasive procedures are planned in the eye or brain or if there is

hemorrhage at these sites.

720 PLEIOTROPY

Leukodepletion of blood components for transfusion, both platelet transfusion and

red blood cell transfusion, reduces alloimmunization and refractoriness but does not

impact on refractoriness due to nonimmune causes

Management of Refractoriness

The patient should be evaluated to detect nonimmune causes of platelet consumption,e.g., infection, splenomegaly, or DIC A serum sample should be tested for HLA antibodies.Testing for platelet-specific alloantibodies and platelet autoantibodies is performed usingthe indirect platelet immunofluorescence test, although this also detects HLA antibodies.Therefore, a test for platelet-specific antibodies, such as the monoclonal antibody-specificimmobilization of platelet antigen assay, should also be used

Immune-mediated platelet refractoriness is managed by providing HLA-matched lets collected by apheresis This is straightforward for patients with common HLA phe-notypes, but there may be few donors for patients with uncommon phenotypes If there

plate-is a good clinical response to the transfusions of HLA-matched platelets, then they areindicated for subsequent transfusions If, however, there is no improvement in the cor-rected posttransfusion count, this might be due to the presence of factors associated withnonimmune platelet consumption or the presence of platelet-specific antibodies Ofpatients with HLA antibodies, 70% respond normally to random platelet transfusion, sothe presence of antibodies is not a diagnostic test If matching for ABO, HLA, and HPAantigens is unsuccessful, platelet cross matching may be helpful in identifying compatibledonors in some of these cases and may be used as the primary strategy for the selection

of compatible platelets Nonimmune-platelet refractoriness is managed by treating theunderlying cause and by increasing the dose of platelets

PLEIOTROPY

The ability of allelic substitutions at a gene locus, or of a cell product, to affect or to be involved in the development of more than one aspect of a phenotype Many protein

kinases are pleiotropic

PLEOMORPHIC SMALL, MEDIUM, AND LARGE T-CELL HTLV-1 + LYMPHOMA

See Adult T-cell leukemia/lymphoma; Non-Hodgkin lymphoma.

Polyneuropathy, Organomegaly, Endocrinopathy, Monoclonal Gammopathy, and Skin

changes (See Osteosclerotic myeloma.)

POLYAGGLUTINABILITY 721

POIKILOCYTOSIS

Any change of red blood cell morphology where the cells display an abnormal shape (see

Peripheral-blood film examination) It occurs in healthy subjects at high altitudes and

frequently in any severe anemia The abnormality in shape may suggest a diagnosis, e.g.,

sickle cell disorder , red blood cell fragmentation syndrome, acanthocytes, or a group of diagnoses, e.g., teardrop poikilocytes.

limitation of expertise in microscopy restricts its performance

The requirements for quality and competence are as important as for testing in a centrallaboratory, but POCT brings its own set of specific problems, such as:

Evaluation of POCT instruments and systems

Purchase and installation of equipment

Maintenance of equipment, consumable supplies, and reagents

Training, certification, and recertification of POCT system operators

Quality control and quality assurance

Once these limitations have been met and overcome, POCT offers distinct advantages

in faster decision making by the physician and consequently earlier commencement oftreatment Improved convenience for the patient may increase compliance and give greater

patient satisfaction POCT is, however, a less economic form of testing Quality

manage-ment is a particular problem because treatment is likely to be initiated before any checks

to ensure the reliability of test results can be applied

POKEWEED MITOGEN

(PWM) A lectin from the root of Phytolacca americana, which is mitogenic to both B and

T-lymphocytes PWM binds to carbohydrate molecules on the lymphocyte In vitro, PWM

causes activation of B-lymphocytes and terminal differentiation, ultimately resulting in

the production of polyclonal immunoglobulin However, it is thought that T-lymphocytes

are necessary for B-lymphocyte activation by PWM to occur T-lymphocyte activationcauses increased cellular cytotoxicity

Various assays using PWM have been developed and are now widely used to investigate

immunodeficiency disorders, e.g., human immunodeficiency virus (HIV) infection.

POLYAGGLUTINABILITY

A property of red blood cells (RBCs) whereby they are usually agglutinated by normal

adult human plasma The usual cause is the transient action, in vivo or in vitro, of bacterial

enzymes, which expose carbohydrate structures on the RBC membrane that are normally

mem-Congenital dyserythropoietic anemias) Lectins can be used serologically to determine the

nature of the polyagglutinability Transfusion of plasma to patients (particularly infants),where the RBCs are polyagglutinable, may give rise to blood transfusion complications Inthese circumstances, only plasma-depleted blood products (e.g., washed RBCs) should betransfused Erroneous blood groups in pretransfusion testing can occur if the test RBCs arepolyagglutinable, but most monoclonal antibodies do not cause problems

POLYARTERITIS NODOSA

A multisystem arteritis particularly affecting small- and medium-sized vessels155 nally described in 1866 by Kussmaul and Maier) Histologically, all three layers of theblood vessel are affected with an inflammatory cell infiltrate Early lesions reveal intimaledema, swelling of muscle fibers, and fibrinoid necrosis resulting in luminal narrowing.Later lesions consist of intimal proliferation and fibrosis with lymphocyte and plasma cellinfiltration Ultimately, there is aneurysm formation, thrombosis, ischemia, and infarction.The lesions often start at arterial bifurcations and spread distally

(origi-The cause is unknown, but 25 to 40% of patients are hepatitis-B surface-antigen positive,

and immune complex deposition has been suggested, antineutrophilic cytoplasmic

antibod-ies being detected There is also a high incidence among patients with hairy cell leukemia.

Clinical Features

Polyarteritis typically affects middle-aged males (M:F ratio 3:1), but all ages can be affected.Any artery or segment of artery can become involved, but commonly, arteries of the kidney,heart, gastrointestinal tract, liver, nerves, muscle, and skin are most commonly affected.Onset is often insidious, with low-grade fever, malaise, anorexia, weight loss, myalgia,and arthralgia

The kidney is the most frequently affected organ (80%), and renal complications are thecommonest cause of death Uremia, proteinurea, hematuria, and hypertension are usual,with aneurysms and infarction of the main renal artery or arcuate and interlobular arteriescommonly seen Acute necrotizing glomerulonephritis is present in 60%

Cardiac involvement (70 to 80%) is the second commonest cause of death due to cardial infarction, acute pericarditis, hypertensive heart disease, and arrhythmias.Gastrointestinal involvement (60 to 70%) manifests as bleeding, ischemia, or bowelperforation Involvement of the nervous system may be peripheral (mononeuritis multi-plex) due to involvement of the vasa nervorum or central (subarachnoid hemorrhage,cerebral infarction, etc.) Clinically detectable skin nodules are only found in 10 to 15% ofpatients, but various other skin manifestations, e.g., urticaria, livedo reticularis, and digitalinfarctions, are more common

myo-Laboratory Features

Normochromic, normocytic anemia with neutrophilia is usual, sometimes with

throm-bocytosis There is polyclonal gammopathy and hypocomplementemia Antinuclear factor(ANF) is positive in 10 to 30% and rheumatoid factor (RF) in 30%, but usually only at low

titers The erythrocyte sedimentation rate (ESR) is usually elevated.

POLYCYTHEMIA RUBRA VERA 723

Definitive diagnosis depends on an adequate tissue biopsy, although angiography isuseful if biopsy is too hazardous

Variants

Microscopic angiitis: a disorder predominantly limited to the renal vessels without

granuloma formation P-ANCA is often found

Churg-Strauss syndrome: a widespread angiitis with granuloma nodules giving rise

to purpura and necrotizing arteritis with eosinophilic infiltrate with late renalinvolvement Early symptoms include asthma and fever associated with anemia

and eosinophilia.

Treatment and Prognosis

Untreated, the prognosis is extremely poor, but high-dose corticosteroids with additional immunosuppressive agents, e.g., azathioprine and cyclophosphamide, have markedly

improved survival Renal failure and hypertension should be treated with conventionaltherapies The overall five-year survival is now 80%

See Erythrocytosis; Polycythemia rubra vera.

POLYCYTHEMIA RUBRA VERA

(PRV) A chronic clonal hematopoietic stem cell disorder characterized by overproduction

of morphologically normal red blood cells, white blood cells, and platelets in the absence

of a definable cause and with suppression of normal polyclonal hematopoiesis

Pathogenesis

There are no known predisposing factors but a single somatic activating point mutation in

the JAK2 gene has been found in many patients (see Cell-signal transduction) There is a

1.2:1 male preponderance, with a median age of onset of 61 years The condition appears to

be more common in Jews and uncommon in blacks, and the incidence in relatives is higherthan in the general population

Erythrocytosis distinguishes this disorder from chronic myeloproliferative disorders,

idio-pathic myelofibrosis and essential thrombocytosis Although erythropoietin-independent

724 POLYCYTHEMIA RUBRA VERA

in vitro erythroid colony formation is a feature of the disease, this abnormality is not specific

for the disorder and is not found in the majority of polycythemia vera erythroid progenitorcells Microvascular thrombosis occurs due to interaction between activated platelets andendothelial cells, and macrovascular thrombosis occurs due to laminar flow disturbance bythe elevated red cell mass, activated platelets, and thrombin generation

Clinical Features

Onset is usually insidious, with nonspecific symptomatology such as malaise, headache,

dizziness, distressing pruritus, extremity pain, or some form of thrombosis

Cerebrovas-cular accidents, myocardial infarction, deep venous thrombosis, pulmonary embolism,and hepatic or portal vein thrombosis are well recognized, with the latter events particu-

larly common in women Hemorrhage and bruising occur in up to 25% of cases, with

gastrointestinal bleeding particularly troublesome, as PRV patients have up to four timesthe incidence of peptic ulceration

Splenomegaly may not be present at onset, but it develops in most patients and can bemassive; hepatomegaly is less common Gout is seen in 10% of cases, although 50 to 70%

of patients have hyperuricemia.

Laboratory Features

Red blood cell morphology is usually normal, but microcytes (due to iron deficiency secondary to hemorrhage) or macrocytes (due to folic acid deficiency) can occur Hemo-

globin F may be slightly raised Granulocytosis (12 to 30 × 109/l) that is left-shifted occurs

in 25 to 60% of patients Basophilia and eosinophilia are also not unusual The NAP score

is high in 70% of patients Thrombocytosis (500 to 2000 × 109/l) is common (80%), andplatelet aggregation may be abnormal in response to epinephrine/adrenaline Plateletmorphology may be bizarre, with macrothrombocytes not uncommon

Bone marrow examination usually reveals trilineage hematopoietic cell hyperplasia but

may be normal Myelofibrosis can be present at onset or develop as part of the natural history of the disease It has no prognostic significance Cytogenetic analysis shows

abnormality in up to 25%, with trisomy 8, trisomy 9, and 20q the most common findings

Diagnosis 184

Absolute erythrocytosis as established by determination of red cell mass and plasma

volume is mandatory for establishing the diagnosis (see Blood volume) The quartet of

erythrocytosis, granulocytosis, thrombocytosis, and splenomegaly makes the diagnosissimple, but in 40% of cases, all of these classical features are not seen A scoring systemhas therefore been proposed by the Polycythemia Vera Study Group for diagnosing PRV(see Table 130),417 but many patients initially lack all the necessary diagnostic features,particularly the red cell mass A new evolving approach to the diagnosis includes thefollowing characteristics: a hemoglobin and hematocrit of greater than 95% confidencelimits; polycythemia rubra vera-related features: thrombocytosis, leukocytosis, microcy-tosis, splenomegaly, pruritus, erythromelalgia, and unusual thromboses; and abnormalbone marrow The serum erythropoietin level, if elevated or very low, can differentiate

PRV from secondary erythrocytosis Newer tests, such as PRV-1 mRNA overexpression or impaired Mpl expression, have yet to be clinically validated There are a number of patients

in whom it is impossible to initially definitely make the diagnosis of PRV, and the term

“idiopathic erythrocytosis” is appropriate in this situation

POLYMORPHIC RETICULOSIS 725

Treatment

There is no known cure for this chronic disease, and therapy is therefore aimed at trolling the consequences of the overactive bone marrow and extramedullary hematopoie-

con-sis Keeping the packed-cell volume (PCV) <0.45 l/l in men and <0.42 l/l in women by

phlebotomy will prevent thrombotic events Asymptomatic thrombocytosis requires notreatment Interferon- and hydroxyurea can be used to control leukocytosis or thrombocy-tosis and extramedullary hematopoiesis.417 Anagrelide is also useful in controlling throm-bocytosis

PUVA light therapy , interferon-αααα, or hydroxyurea can be used for intractable pruritus

if antihistamines or antidepressants are not effective, and allopurinol will control

hyper-uricemia Epsilon aminocaproic acid or tranexamic acid can control platelet-related orrhage, while aspirin can alleviate erythromelalgia In some patients,418 splenectomy may

hem-be necessary for intractable splenomegaly

Prognosis

The natural history of polycythemia is variable because the disease is not monolithic In

a few patients, the disease is rapidly progressive, but most can look forward to manydecades of productive life Myelofibrosis is a feature of the natural history of PRV but isnot itself a prognostic marker

POLYMERASE CHAIN REACTION

See Molecular genetic analysis.

POLYMORPHIC LYMPHOID PROLIFERATION

See Posttransplant lymphoproliferative disorders.

POLYMORPHIC RETICULOSIS

See Extranodal NK/T-cell lymphoma, nasal type; Non-Hodgkin lymphoma.

TABLE 130

Diagnostic Scoring System for Polycythemia Rubra Vera

1 Red cell mass Male >36 ml/kg Female >32 ml/kg

Note: PRV is diagnosed if: (a) all three major criteria are present or (b)

the first two of the major criteria plus any two minor criteria are present.

726 POLYMORPHONUCLEAR NEUTROPHIL

POLYMORPHONUCLEAR NEUTROPHIL

See Neutrophils.

POLYMYALGIA RHEUMATICA

(Polymyositis; giant-cell arteritis; temporal arteritis) The hematological changes occurring

in patients with polymyalgia rheumatica and closely related giant-cell arteritis.419 Thesechanges include:

Erythrocyte sedimentation rate elevated to a marked degree in most, but in a few it

is normal

Plasma viscosity raised

C-reactive protein raised

Polyclonal hypergammaglobulinemia

Rouleaux of red blood cells in peripheral-blood smears

Anemia of chronic disorders

POLYMYOSITIS

An inflammatory disorder of striated muscle causing proximal muscle weakness In some

patients there is involvement of the skin — dermatomyositis Persons with human

leu-kocyte antigens HLA-B8/DR 3 are genetically predisposed The associated hematologicalchanges are:

Mild anemia of chronic disorders

Leukocytosis with eosinophilia

Erythrocyte sedimentation rate markedly elevated

POLYNEUROPATHY, ORGANOMEGALY, ENDOCRINOPATHY, MONOCLONAL GAMMOPATHY, AND SKIN CHANGES

(POEMS syndrome) See Osteosclerotic myeloma.

POORLY DIFFERENTIATED LYMPHOCYTIC LYMPHOMA

See Non-Hodgkin lymphoma.

PORCINE FACTOR VIII

See Coagulation factor concentrates.

PORPHORINURIA

See Porphyrias.

PORPHYRIA CUTANEA TARDA

See also Porphyrias.

PORPHYRIAS 727

The clinical disorder resulting from an autosomally dominant partial deficiency of thehepatic uroporphyrinogen decarboxylase; it is the most common of the porphyrias Thecharacteristic clinical features are: photosensitivity with vesiculo-bullous eruptions,mainly on the hands and face; hyperpigmentation; and excessive facial hair growth Severescarring of the skin may suggest scleroderma There is excessive tissue iron loading withelevation of the serum transferrin saturation and ferritin levels and hepatic siderosis inmany patients that sometimes cause hepatic cirrhosis Many patients have common muta-

tions of the hemochromatosis-associated HFE gene on Ch6p (especially HFE C282Y or

H63D 104) The severity of iron overload can be estimated by analyses of a liver specimen

obtained by biopsy or by quantitative phlebotomy The urinary excretion of

uroporphy-rinogens I and III and, to a lesser extent, coproporphyrinogen are increased (Table 131).All patients should abstain from alcohol ingestion or exposure to benzene-related com-pounds, especially those encountered in industrial settings In women, estrogen or proges-terone therapy can exacerbate porphyria cutanea tarda Photosensitivity resolves in manypatients who achieve iron depletion with therapeutic phlebotomy Although the mecha-nism of action is unclear, therapy with chloroquine may be helpful in phlebotomy-resistantpatients Chloroquine is concentrated in the liver and may complex with the porphyrin

or stimulate its release from hepatocytes

PORPHYRIAS

A group of metabolic disorders involving the specific enzymes of the heme biosynthetic

pathway These disorders usually have a genetic basis, although the enzyme deficiency

in porphyria cutanea tarda may be either inherited or acquired Porphyrias are ized biochemically by patterns of accumulation and excretion of intermediates or oxidizedintermediates of the heme biosynthetic pathway The enzyme deficiencies do not pro-foundly impair total cellular heme synthesis and content Rates of heme synthesis areusually normal and may even be increased in some patients with porphyria

character-TABLE 131

Laboratory Features of the Porphyrias

ALA synthase deficiency

Isocopro Hepatoerythropoietic

porphyria

Isocopro

Note: ALA = aminolevulinic acid; 7-Carboxyl = 7-carboxylporphyrin; Copro = coproporphyrin; Isocopro =

isocoproporphyrin; PBG = porphobilimogen; Proto = protoporphyrin; Uro = uroporphyrin; ZnPP = zinc protoporphyrin.

Source: Modified from Sassa, S., The hematologic aspects of porphyria, in Williams Hematology, 6th ed., Beutler,

E., Lichtman, M.A., Coller, B.S., Kipps, T.J., and Seligsohn, U., Eds., McGraw-Hill, New York, 2001.

728 PORPHYRIAS

Many symptoms of porphyria are nonspecific and mimic those of more common ders The major manifestation in some patients is cutaneous photosensitivity In somecases there are poorly understood effects on the nervous system that can be life threatening,but these manifestations occur only in those types of porphyria in which porphyrinprecursors accumulate The diagnosis rests on a high level of suspicion and appropriatelaboratory testing

disor-Classification

Porphyrias are divided into erythropoietic and hepatic types, depending on whether the

excess production of porphyrin precursors and porphyrins occurs in the red blood cells

or liver, although some porphyrias possess both “erythroid” and “hepatic” features.

Porphyrias with neurovisceral features are also called acute porphyrias, and those withcutaneous photosensitivity are called cutaneous porphyrias Latent forms probably exist

in all types A specific type of porphyria is associated with specific red blood cell enzymedeficiencies (see Table 132) Chromosomal locations for several of the enzymes have beendefined.172

In acute porphyrias, the effects of drugs are most important (see Table 133) In affectedpatients, life-threatening attacks of porphyria can occur after exposure to certain com-monly prescribed drugs Many individual drugs have been categorized as unsafe because

they have caused acute attacks in humans or are porphyrinogenic in animals or in in vitro systems (see British Natural Formulary, 51, 499, 2006).

Biochemical Diagnosis 420

In porphyrias, porphyrins accumulate in normoblasts, skin, and liver Their presence can

be detected by ultraviolet light, birefringency of protoporphyrin crystals under polarizedlight, fluorescence microscopy, or electron microscopy Diagnosis is based on patterns ofaccumulation and excretion of the intermediates of the heme biosynthetic pathway in redblood cells, urine, and feces Table 131 summarizes the patterns in the different types ofporphyria

Physicochemical Properties of the Porphyrins

Figure 99 shows the structural formula of the porphyrin nucleus and gives a diagrammaticrepresentation of some naturally occurring porphyrins

TABLE 132

Association of Enzyme Deficiency with Clinical Porphyria

Uroporphyrinogen III cosynthase (partial) Congenital erythropoietic porphyria (E)

Uroporphyrinogen III cosynthase (severe) Hepatoerythropoietic porphyria

a H = hepatic form; E = erythroid form.

PORPHYRIAS 729

The porphyrin molecule is formed by four pyrrole rings joined by four methane bridges.The tetrapyrrole can have two to eight carboxylic side chains If each pyrrole ring containstwo different acidic side chains, acetyl and propionyl, there are four possible isomers (I,

II, III, and IV) The asymmetric type III isomer and the symmetric type I isomer occurmost frequently in nature Uroporphyrin is water soluble and is excreted predominantly

by the kidney In contrast, protoporphyrin is very hydrophobic and is eliminated byhepatobiliary excretion into the intestinal tract Coproporphyrin has intermediate properties.The electronic configuration of tetrapyrroles favors the absorption of radiant energy byporphyrins The violet spectrum of the Soret band (400 to 410 nm) is the region of greatestenergy absorbance A lesser degree of absorbance occurs in the visible band (580 to 650nm) Absorbing radiant energy results in raising orbital electrons from their ground state

to singlet and triplet excited-state energy levels Single excited molecules are short-livedand emit fluorescence, whereas triplet excited porphyrins are long-lived and emit phos-phorescence Triplet excited porphyrins possess sufficient energy to convert dissolvedmolecular oxygen to singlet oxygen, the main cytotoxic event in photocytotoxicity and inphotodynamic therapy

When porphyrins are increased in the blood, the patient’s skin becomes very sensitive

to sunlight, and blisters form readily Cutaneous lesions include:

TABLE 133

Drug Groups Associated with Acute Porphyria

Individual Drugs Associated with Acute Porphyria

Source: British National Formulary 51, 9.8.2, 2006, p 503 and updated twice per year With permission.

com-Clinical Features and Treatment

These depend upon the form of the porphyria and must be considered separately for eachtype:

Congenital erythropoietic porphyria

Hereditary coproporphyria

FIGURE 99

Structural formula of the porphyrin nucleus and diagrammatic representation of some naturally occurring

porphyrins (Prepared by G.E Cartwright; reproduced from Wintrobe, M.M., Clinical Hematology, 6th ed., Henry

Kimpton, London, 1967 With permission.)