Lead-time bias — MRCPsych Paper A MCQ

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ModerateEpidemiology and research methodsLead-time biasMRCPsych Paper A

A memory-clinic screening programme detects dementia earlier than routine clinical presentation. People diagnosed through screening have longer survival measured from the date of diagnosis, but their mean age at death and all-cause mortality are unchanged. Which phenomenon most likely explains the apparent survival benefit?

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Correct answer: E — Lead-time bias

The correct answer is E, lead-time bias. Screening moves the recorded time of diagnosis earlier, so survival measured from diagnosis becomes longer even when the time of death and mortality are unchanged. Length-time bias instead occurs because screening preferentially detects slowly progressive disease with a longer detectable preclinical phase. Confounding by severity and volunteer selection bias could create differences between screened and unscreened groups, but neither specifically explains longer diagnosis-to-death survival with unchanged age at death. A type II error is failure to detect a genuine difference and does not systematically create an apparent survival advantage.

Reference: Andrade C. Lead Time Bias in Medicine and Psychiatry: A Concept Simply Explained. Indian Journal of Psychological Medicine. 2025;47(3):295–296. https://pubmed.ncbi.nlm.nih.gov/40162435/