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Lead-time bias — USMLE Step 2 CK MCQ

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HardPreventive Medicine & BiostatisticsLead-time biasUSMLE Step 2 CK

A screening program identifies an indolent cancer a mean of 4 years before it would have become symptomatic. Five-year survival measured from diagnosis is 88% among screen-detected cases and 54% among clinically detected cases, but randomized groups have identical disease-specific mortality. Which phenomenon best explains the apparent survival advantage?

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Reveal the answer and explanation

Correct answer: B — Lead-time bias from starting the survival clock earlier

Explanation lettering: E = shown as A · D = shown as B · B = shown as C · A = shown as D · C = shown as E

D is correct: diagnosis occurs about 4 years earlier, so measured survival from diagnosis lengthens even though the time of death and disease-specific mortality do not change. A can also make screen-detected cancers appear favorable but depends on preferential sampling of slower-growing lesions rather than merely advancing the diagnostic date. B adds cases that would never cause symptoms or death. C reflects baseline differences between screened and unscreened volunteers. E concerns retrospective exposure reporting and does not explain the stated randomized mortality pattern.

Reference: NCI Dictionary of Cancer Terms: lead-time bias. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/lead-time-bias