A screening test has sensitivity 80% and specificity 90%. It is applied to a population with disease prevalence of 1%. Approximately, what is the Positive Predictive Value (PPV)?
- A 7.5% ✓
- B 80%
- C 90%
- D 50%
Correct answer: A. 7.5%
Explanation
Using a 2×2 table with 10,000 people: prevalence 1% = 100 diseased, 9,900 non-diseased. True positives = 80% of 100 = 80. False positives = 10% of 9,900 = 990. PPV = 80/(80+990) = 80/1070 ≈ 7.5%. This illustrates the critical impact of prevalence on PPV — even a highly specific test generates many false positives in low-prevalence populations, making positive results less reliable. This is the basis for selective (targeted) rather than universal screening in low-prevalence conditions.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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