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Positive predictive value — USMLE Step 3 MCQ

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ModeratePreventive Medicine, Epidemiology, BiostatisticsPositive predictive valueUSMLE Step 3

A screening test has sensitivity 90% and specificity 90%. It is used in a population where disease prevalence is 1%. A physician notices that many positive results are false positives compared with use in a high-risk clinic. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: B — Low disease prevalence reduces positive predictive value

Positive predictive value decreases when prevalence is low, even when sensitivity and specificity are unchanged.

Reference: Users' Guides to the Medical Literature: Diagnostic Tests