A screening test for colorectal cancer has a sensitivity of 80% and specificity of 90%. It is applied to a population where disease prevalence is 1%. Approximately what proportion of individuals with a positive test will actually have the disease?
- A About 89%
- B About 44%
- C About 7% ✓
- D About 97%
Correct answer: C. About 7%
Explanation
In 10,000 people, 100 have disease: true positives equal 80. Of the 9,900 without disease, false positives equal 990. Positive predictive value equals 80 divided by 1,070, roughly 7.5%. The key teaching point is that PPV falls sharply at low prevalence even with good sensitivity and specificity. Option B would be obtained if prevalence were around 10%, illustrating how strongly PPV depends on pre-test probability.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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