A rapid screening test for HIV has sensitivity 90% and specificity 95%. It is applied to a blood donor population where HIV prevalence is 2%. The Negative Predictive Value of this test is approximately:
- A 99.8% ✓
- B 95.0%
- C 90.5%
- D 26.7%
Correct answer: A. 99.8%
Explanation
In 10,000 donors, 200 are infected: true positives 180, false negatives 20. Of 9,800 uninfected, true negatives are 95% of 9800 = 9,310. NPV = 9310 divided by (9310 + 20) = 99.8%. Option D is the PPV (180 out of 680), included because low prevalence makes PPV poor even when NPV is near perfect. This asymmetry explains why confirmatory testing follows every reactive screen.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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