A new diagnostic test for tuberculosis is evaluated against sputum culture as the gold standard. The test yields the following results: sensitivity 90%, specificity 85%, disease prevalence 5% in the screened population. What is the positive predictive value?
- A 24% ✓
- B 90%
- C 85%
- D 95%
Correct answer: A. 24%
Explanation
Using a hypothetical population of 1000: 50 have TB (5% prevalence). Sensitivity 90% gives 45 true positives. Of 950 without TB, specificity 85% gives 807.5 true negatives and 142.5 false positives. PVP = TP/(TP+FP) = 45/(45+142.5) = 24%. PVP depends heavily on prevalence; even with good sensitivity and specificity, low prevalence yields a low PPV. This is a fundamental concept in screening.
Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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