Positive predictive value and prevalence — MRCPCH TAS MCQ

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HardStatistics and evidence-based medicinePositive predictive value and prevalenceMRCPCH TAS

A diagnostic test has unchanged sensitivity and specificity. It is moved from a tertiary immunology clinic to population screening, where disease prevalence is far lower. What happens to the positive predictive value?

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Correct answer: E — It decreases

Positive predictive value falls when prevalence falls, even if sensitivity and specificity are unchanged. Sensitivity and specificity are test properties, whereas predictive values depend heavily on pre-test probability. The pearl is that low-prevalence screening programmes can generate many false positive results.

Reference: The Science of Paediatrics MRCPCH Mastercourse; CEBM diagnostic test principles; RCPCH Theory examination syllabi