Pediatric emergency medicine trisk 608

Test Laboratory abnormalitymetabolic diseases a Indications, comments Blood CBC plasma Neutropenia ± vacuoles, anemia, and/or thrombocytopenia Organic acidemias Urea cycle defects Carboh

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TABLE 95.4

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Test Laboratory abnormality

metabolic diseases a

Indications, comments

Blood

CBC (plasma) Neutropenia (± vacuoles),

anemia, and/or thrombocytopenia Organic acidemias Urea cycle defects Carbohydrate intolerance disorders Carbohydrate

production/utilization disorders

Lysosomal storage disorders Mitochondrial disorders

Neutropenia may be masked by infection Patients with certain IEMs are at increased risk of infection; infection can also precipitate metabolic crisis Anemia hemolytic, megaloblastic, or normocytic, depending on specific IEM

Glucose (serum) Hypoglycemia

Aminoacidopathies Organic acidemias Fatty acid oxidation defects

Carbohydrate intolerance disorders Carbohydrate

Mitochondrial disorders

Hypoglycemia may be due to primary defect of gluconeogenesis or glucose consumption that exceeds production

Test of acid–base

status (serum)

Electrolytes

Anion gap

pH (arterial or

venous)

Primary metabolic acidosis

Aminoacidopathies Organic acidemias Fatty acid oxidation defects

Mitochondrial disorders Primary respiratory alkalosis

Urea cycle defects

Na+ , K+ , Cl− usually normal unless abnormal secondary to vomiting, which may produce hyperchloremic metabolic acidosis, or to rhabdomyolysis, which may result in hyperkalemia Normal bicarbonate does not rule out amino or organic acidemias

Ammonia (plasma) Hyperammonemia

Aminoacidopathies Organic acidemias Urea cycle defects

Obtain if altered consciousness, persistent or recurrent unexplained vomiting, recurrent dizziness or ataxia, primary metabolic acidosis with increased anion gap,

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Fatty acid oxidation defects

primary respiratory alkalosis in the absence of toxic ingestion

Must be free-flow venous (no tourniquet) or arterial Arterial preferred because skeletal muscle releases ammonia, ice sample immediately, assay promptly Newborns 90–150 μg/dL, children 40–120 μg/dL, adults 18–54 μg/dL

( www.pediatriccareonline.org/pco/ub/view/Pediatric- drug-Lookup/153930/0/Normal-Laboratory-Values-for-Children )

Normal <100 μg/dL, neonate <80 μg/dL>1 mo False positives—valproic acid

Liver function tests

(serum)

Bilirubin

Transaminases

Clotting factors

Hyperbilirubinemia Aminoacidopathies (tyrosinemia) Carbohydrate intolerance disorders Elevated transaminases Aminoacidopathies Organic acidemias Urea cycle defects Fatty acid oxidation defects

Lysosomal storage disorders Mitochondrial disorders Peroxisomal disorders

Obtain if vomiting, jaundice, and/or hepatomegaly Hyperbilirubinemia predominantly conjugated, except galactosemia first few days may be unconjugated

Muscle function

tests (serum)

Lactate

dehydrogenase

Aldolase

Creatine kinase

Abnormal muscle enzymes Carbohydrate production/utilization disorders

Fatty acid oxidation defects

Obtain if muscle weakness, tenderness, cramping, atrophy, exercise intolerance

Carnitine deficiency due to carnitine transport disorders or secondary to organic acidemias, fatty acid oxidation defects

Urine

Reducing

substances

(Clinitest)

Aminoacidopathies (tyrosinemia, alkaptonuria) Carbohydrate intolerance disorders

Clinitest positive for reducing substances and dipstick negative for glucose (glucose oxidase reaction) False positives—penicillins, salicylates, ascorbic acid, drugs excreted as glucuronides

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Absence of reducing substances does not eliminate possibility of IEM

Ketones (Ketostix,

Acetest)

Elevated ketones Aminoacidopathies Organic acidemias Carbohydrate intolerance disorders Carbohydrate

Mitochondrial disorders Absent ketones,

hypoketosis Fatty acid oxidation defects

Ketones detected by Ketostix, Chemstix, Acetest Inappropriate ketones

Ketonuria in neonates Ketonuria, normal glucose beyond neonate Low/absent ketones, hypoglycemia beyond neonate

Organic acidemias Carbohydrate production/utilization disorders

Not always present, even with rhabdomyolysis, especially if creatinine kinase <10,000 IU

aWithin disease categories, not all diseases have the laboratory abnormality In disorders of protein metabolism, carbohydrate metabolism and fatty acid oxidation defects and abnormality may be present only during acute crisis.

IEM, inborn error of metabolism.

Adapted from Weiner DL Inborn errors of metabolism In: Aghababian RV, ed Emergency Medicine: The Core Curriculum Philadelphia,

PA: Lippincott-Raven; 1999:705.

Hypoglycemia Serum glucose level of less than 40 mg/dL in the neonate and less than 50 mg/dL

beyond the neonatal period should be considered abnormally low Even with poor oral intake and/or metabolic stressors, hypoglycemia less than 45 mg/dL is unusual in the normal child Hypoglycemia may cause a decreased level of consciousness, irritability, and seizures Newborns may also have a high-pitched cry, hypothermia, cyanosis, and poor feeding In the older child or adult, symptoms may include headache, blurred vision, repeated yawning, diaphoresis, pallor, and nervousness Hypoglycemia most commonly occurs with fatty acid oxidation defects, disorders of carbohydrate metabolism, and hyperinsulinemic states Low serum glucose can also be seen with aminoacidopathies and organic acidemias due to inhibition of hepatic gluconeogenesis in these disorders In patients with hypoglycemia, absence of ketonuria is highly suggestive of a fatty acid oxidation defect On the other hand, neonates should never have ketonuria, and when present, it is suggestive of an IEM Beyond the neonatal period, hypoglycemia with inappropriately low or absent ketones is also always abnormal The presence of urinary ketones in a patient with hypoglycemia does not rule out an IEM, particularly short-chain fatty acid oxidation defects, organic acidemias, disorders of carbohydrate metabolism, or ketotic hypoglycemia

of childhood Hypoketosis, if not evident from the urine, can be determined by measuring ketones (3-hydroxybutyrate and acetoacetate) and free fatty acids in blood In patients with hypoglycemia, the following laboratory studies should be sent: plasma amino acids, acylcarnitine, urine organic acids and acylglycines, serum cortisol, insulin, liver function tests (LFTs), and ammonia Growth hormone is not an informative test in the acute setting Causes of hypoglycemia other than IEM include liver disease; hyperinsulinemia; toxic ingestions of salicylates, β-blockers, ethanol, or polyethylene glycol; maternal diabetes/gestational diabetes; prematurity or small for gestational age; asphyxia; and sepsis

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