A new screening test for diabetes has a sensitivity of 80% and a specificity of 90%. It is applied to a population where the prevalence of diabetes is 2%. What is the approximate positive predictive value of the test?

• A About 89%
• B About 47%
• C About 80%
• D About 14% ✓

Explanation

In 10,000 people, 200 have diabetes: 160 are true positives (80% sensitivity). Of the 9,800 non-diabetics, 980 are false positives (10% of them). PPV = 160 divided by (160 + 980) = approximately 14%. The key teaching point is that PPV falls sharply at low disease prevalence even with good sensitivity and specificity. Option A (89%) is the negative predictive value trap, and option C confuses PPV with sensitivity.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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