A screening programme detects early-stage prostate cancer and shows a 5-year survival of 85% compared to 55% for clinically-diagnosed cases. However, mortality rates remain unchanged. This discrepancy is BEST explained by:

• A Lead-time bias — survival is measured from earlier diagnosis, not true life extension ✓
• B Selection bias — healthier men volunteer for screening
• C Length-biased sampling — screening preferentially detects slow-growing tumours
• D Information bias — cause of death is misclassified

Explanation

Lead-time bias occurs when screening advances the time of diagnosis without extending actual lifespan; survival appears longer because the clock starts earlier (at diagnosis) rather than at symptom onset, but total lifespan is unchanged. This explains apparently improved 5-year survival without any change in mortality rates — the gold standard for assessing true benefit. Length-biased sampling (preferential detection of slow-growing cancers) is a distinct but related phenomenon. Selection bias and information bias do not specifically explain unchanged mortality with improved survival.

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.