Lead-time bias in cancer screening studies causes which of the following distortions in outcome measurement?

• A Screen-detected cases appear to survive longer because diagnosis was advanced in time, not because death was postponed ✓
• B Slowly growing tumours are preferentially detected, making screening appear more effective than it is
• C Patients with screen-detected cancers have healthier baseline characteristics than unscreened patients
• D Patients enrolled in screening programmes are more likely to comply with treatment

Explanation

Lead-time bias occurs when screening advances the date of diagnosis without changing the date of death. Survival time (from diagnosis to death) appears longer in screen-detected cases purely because the clock started earlier, not because the patient lived longer in absolute terms. This inflates 5-year survival rates in screened populations. Option B describes length-biased sampling (overdetection of slow-growing, indolent tumours that are present longer in the preclinical detectable phase). Option C describes healthy screenee bias (healthy user bias). Option D describes compliance or adherence bias — a form of selection bias affecting RCTs.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.