Positive predictive value and prevalence — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

HardStatistics and EBMPositive predictive value and prevalenceFRCA Primary

A rare anaesthetic complication is screened using a test with good sensitivity and specificity. When the same test is used in a lower-risk population, positive results are less reliable. Which measure has fallen most?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Positive predictive value

Positive predictive value falls as prevalence falls, even when sensitivity and specificity remain unchanged. Sensitivity and specificity are intrinsic test characteristics in the same setting, while predictive values depend on pre-test probability. Likelihood ratios are less prevalence-dependent than predictive values. This is why low-risk screening can generate many false positives.

Reference: Primary FRCA statistics texts; Users' Guides to the Medical Literature