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Specificity — USMLE Step 3 MCQ

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HardPreventive Medicine, Epidemiology, BiostatisticsSpecificityUSMLE Step 3

A diagnostic test is applied to 2,000 people in a setting where disease prevalence is 5%. Sensitivity is 90% and specificity is 95%. Assuming these values apply, what is the expected positive predictive value?

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Correct answer: A — Approximately 49%

The best answer is “Approximately 49%”. Of 100 people with disease, 90 test positive. Of 1,900 without disease, 5%—95 people—test false positive. Positive predictive value is therefore 90/(90 + 95) = 48.6%, approximately 49%. It cannot be read directly from sensitivity or specificity because prevalence determines the false-positive burden.

Reference: CDC Principles of Epidemiology in Public Health Practice: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/SS1978.pdf