In a 2×2 contingency table for a screening test, the test correctly identifies 90 of 100 true cases (diseased) and correctly excludes 135 of 150 non-cases. Compute the positive predictive value if the test is used in a population with 40% disease prevalence.
- A 72%
- B 75%
- C 80% ✓
- D 85%
Correct answer: C. 80%
Explanation
Sensitivity = 90/100 = 90%; Specificity = 135/150 = 90%. With 40% prevalence in 1000 people: 400 diseased, 600 non-diseased. TP = 400 × 0.90 = 360; FP = 600 × 0.10 = 60. PPV = TP/(TP+FP) = 360/(360+60) = 360/450 = 80%. The high disease prevalence (40%) raises PPV substantially above what would be seen at low prevalence.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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