Bài giảng y học chứng cứ bài 5 chứng cứ của các nghiên cứu chuẩn đoán, trình bày cơ sở chuẩn đoán, đánh giá các nghiên cứu chuẩn đoán, sai lầm trong chuẩn đoán, thiết kế cơ bản của một nghiên cứu chuẩn đoán chính xác

Were all patients subjected to the gold standard’2. Was there an independent, blind or objective comparison with the gold standard’?... You want to find out how good chest X rays are fo

Bai 5: CHUNG CU CUA CAC NGHIÊN CƯU CHAN DOAN

Matthew J Thompson

GP & Senior Clinical Scientist

<, Department of Primary Health Care

University of Oxford

Nội dung bài học

=» Cơ sở chân đoán

=m Đánh giá các nghiên cứu chân đoán

acl hệt các sai lâm trong chân đoán là

ognitive errors:

Conditions of uncertainty

Thinking is pressured Shortcuts are used

(Ann Croskerry Ann Emerg Med 2003)

Những sai lâm trong chan ¢ doan (Diagnostic errors -

The next frontier for Patient Safety Newman- -Toker, JAMA 2009)

airport US hospital deaths from

misdiagnosis per year Adverse events, negligence cases, serious disability more II ikely to be related to

misdiagnosis than drug errors

Diagnostic strategies particularly important

where patients present with variety of conditions

and possible diagnoses

Thi du: nguyén nhan cua ho la gi?

Comprehensive history —~ examination —— differential diagnosis —— final diagnosis

`

Coagralulafieas,

iE only took you

65299 seconds

Measles, Oropharyngeal cancer, Goodpasture's syndrome Pulmonary oedema, Pulmonary embolism, Mycobacterium tuberculosis Foreign body in respiratory tract, Diffuse panbronchiolitis, Bronchogenic carcinoma Broncholithiasis, Pulmonary fibrosis, Pneumocystis carinii

Captopril, Whooping cough, Fasciola hepatica Gastroesophageal reflux, Schistosoma haematobium, Visceral leishmaniasis Enalapril, Pharyngeal pouch, Suppurative otitis media

Upper respiratory tract infection, Arnold's nerve cough syndrome, Allergic bronchopulmonary aspergillosis Chlorine gas, Amyloidosis, Cyclophosphamide

Tropical pulmonary eosinophilia, Simple pulmonary eosinophilia, Sulphur dioxide Tracheolaryngobronchitis, Extrinsic allergic alveolitis, Laryngitis

Fibrosing alveolitis, cryptogenic, Toluene di-isocyanate, Coal worker's pneumoconiosis Lisinopril, Functional disorders, Nitrogen dioxide, Fentany!

Asthma, Omapatrilat, Sinusitis Gabapentin, Cilazapril

dlagnostic reasoning

Danh gia test chan doan

let ke co ban Cua mot nghien cuu chan

doan chinh xac

Validity of diagnostic studies

1 Was an appropriate spectrum of patients

included?

2 Were all patients subjected to the gold standard’?

3 Was there an independent, blind or objective

comparison with the gold standard’?

1 Was an appropriate spectrum of

1 Was an appropriate spectrum of

patients included?

You want to find out how good chest X rays are for diagnosing pneumonia in the Emergency Department

Best = all patients presenting with

difficulty breathing get a chest X-ray Spectrum bias = only those patients in

whom you really suspect pneumonia get a

chest X ray

2 Were all patients subjected to the gold

2 Were all patients subjected to the gold

Standard?

You want to find out how good Is exercise ECG (‘treadmill test’) for identifying patients with angina

The gold standard is angiography Best = all patients get angiography Verification (work-up bias) = only patients who have a positive exercise ECG get

angiography

3 Was there an independent, blind or objective comparison with the gold

3 Was there an independent, blind or objective comparison with the gold

standard’? Observer bias

You want to find out how good Is exercise ECG for identifying patients with angina All patients get the gold standard

(angiography) Observer bias = the Cardiologist who does the angiography Knows what the exercise ECG showed (not blinded)

Differential Reference Bias

RUN ID Ref Std B

Were all patients subjected to the Gold

Standard?

Was there an independent, blind or objective comparison with the Gold Standard?

Go SN

Appraising diagnostic tests '®:

1 Are the results valid?

2 What are the results? mm

3 WIlI they help me look after my patients?

True

neøafives

have a positive test result

a highly sensitive test will not miss many

OIG

2 by 2 table: specificity

Disease

without the disease

who have a negative

Sensitivity = a/at+c False positive rate = b/b+d

(same as 1-specificity)

Your father went to his doctor and was told that his test for a disease was positive He is really

worried, and comes to ask you for help!

After doing some reading, you find that for men

of his age:

The prevalence of the disease is 30%

The test has sensitivity of 50% and specificity of 90%

“Son, tell me what’s the chance

| have this disease?”

Prevalence of 30%, Sensitivity of 50%, Specificity of 90%

positive

of whom 15 have the

disease

So, chance of disease Is

15/22 about 10%

Prevalence of 4%, Sensitivity of 50%, Specificity of 90%

positive

of whom 2 have the disease

So, chance of disease Is

2/11.6 about 17%

Doctors with an average of 14 yrs experience

answers ranged from 1% to 99%

half of them estimating the probability as 50%

Gigerenzer G BMJ 2003;327:741-744

Sensitivity and specificity don't vary

with prevalence

= Test performance can vary in different settings/

patient groups, etc

= Occasionally attributed to differences in disease

prevalence, but more likely is due to differences in diseased and non-diseased spectrums

2 xX 2 table: positive predictive value

2 X 2 table: negative predictive value

with a neøafive tesf

What’s wrong with PPV and TY

= Depend on accuracy of the test and prevalence of the disease

2 X 2 table: positive likelinood ratio

How much more often a positive test occurs in people with

compared to those without the Ïisease

2 X 2 table: negative likelinood ratio

disease compared to those without the disease

—45% 0.1

LR<0.1 = strong neøative fesf result

oS lj

Absence of severe right lower McBurney's point tenderness

quadrant tenderness Rovsing's sign

sence of McBurney's point tenderness Psoas sign

reasoning

Pre test 5%

Probability Ratio Probability

—

t

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ess |) http://www.cebm.net; w| Links ”

^ Sean

ie Control-C to copy selected text, Control-V to paste and Control-X to cut.)

TAME 2 OL\Ir diagnosis

¥velcome to the web site of

Centre for Evidence-Based

in Oxford in the UK

TEST

to apply for bursary Teaching Evidence

Our broad aim is to develop SENSITIVITY Y

and promote evidence-bas

care and provide supporta

resources to doctors and h

care professionals to help

the highest standards of m

Please enter the numbers in each group for the diagnostic testin the study VWhen you're ready, click the

CALC button to work out Sensitivity, Specificity, Likelinood Ratios, etc

Learn more about EBM and mu ý

CEBM Bete - Workshop Videos

sed medicine

by the BMJ bi-monthly It alerts

clinicians to the latest EBM advances

Cind ait mara

The latest FREE tools fore

practice and teaching of

ourvey of 300 US physicians

8 used Bayesian methods, 3 used

ROC curves, 2 used LRs

VWVhy?

Indices unavailable

lack of training

not relevant to setting/population

other factors more important

(Reid et al Academic calculations versus clinical judgements: practicing physicians’ use of quantitative measures of test accuracy Am J Med 1998)

Reproducibility of the test and interpretation in my setting

Jo results apply to the mix of patients | see?

Will the results change my management?

Impact on outcomes that are important to patients?

Where does the test fit into the diagnostic strategy?

Costs to patient/health service?

Kappa = measure of intra- observer reliability

Value of Kappa Strength of Agreement

<0.20 Poor

0.21-0.40 ahve 0.41-0.60 Moderate 0.61-0.80 Good