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Predictive value and prevalence — USMLE Step 3 MCQ

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EasyPreventive Medicine, Epidemiology, BiostatisticsPredictive value and prevalenceUSMLE Step 3

A diagnostic test for bacterial meningitis has sensitivity 95% and specificity 60%. The test is applied in a population with very low meningitis prevalence. Many positive results are false positives despite high sensitivity. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: B — Low pretest probability reduces positive predictive value

Positive predictive value depends strongly on disease prevalence. In low-prevalence settings, even tests with high sensitivity can generate a high proportion of false-positive results when specificity is modest.

Reference: USMLE Step 3 biostatistics content outline