A screening programme detects prostate cancer in asymptomatic men. Five-year survival after diagnosis is much higher among screen-detected patients than among clinically diagnosed patients, yet age-specific prostate cancer mortality is identical in screened and unscreened populations. What best explains the apparent improvement in survival?

• A Confounding by indication
• B Length bias
• C Recall bias
• D Lead-time bias ✓

Explanation

Screening advances the time of diagnosis without delaying the time of death, so survival measured from diagnosis looks longer even when nothing has changed biologically. That is lead-time bias. Length bias refers to screening preferentially detecting slow-growing indolent tumours, which would affect disease composition rather than this pure survival-versus-mortality mismatch. The decisive clue is that mortality is identical while survival from diagnosis improves, which can only be explained by earlier detection.

Reference: Gordis Epidemiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.