In a community screening study, a new diagnostic test is evaluated. Of 100 patients with the disease, 90 test positive. Of 200 truly healthy individuals, 180 test negative. The positive predictive value of this test in a population with 10% disease prevalence is:
- A 75%
- B 50% ✓
- C 90%
- D 81%
Correct answer: B. 50%
Explanation
Sensitivity = 90/100 = 90%; Specificity = 180/200 = 90%. With 10% prevalence in a hypothetical 1000-person population: TP = 0.90×100 = 90, FP = 0.10×900 = 90. PPV = TP/(TP+FP) = 90/180 = 50%. This illustrates how PPV drops dramatically when prevalence is low, even with a highly sensitive and specific test.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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