After introduction of a mammography screening programme, screen-detected breast cancers show markedly better 5-year survival than clinically detected cancers. Critics note that much of this advantage persists even after adjusting for lead time, because screening disproportionately detects slow-growing, less aggressive tumours that spend a long time in the detectable preclinical phase. This phenomenon is termed:

• A Lead time bias
• B Healthy worker effect
• C Overdiagnosis due to observer variation
• D Length bias ✓

Explanation

Length bias arises because indolent tumours remain in the detectable preclinical phase longer, giving screening more opportunity to capture them, while rapidly fatal tumours surface between screening rounds. This inflates the observed survival of screen-detected cases independent of any real benefit. Lead time bias shifts the start point of measurement, whereas here the issue is selective detection of slower disease, which is the defining feature of length bias.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.