A new screening programme detects prostate cancer at age 62 years in a man who would have been diagnosed symptomatically at age 68 years under usual care. He dies at age 72 years. An enthusiastic report claims screening added 6 years of survival. The flaw in this claim is best described as:

• A Length bias
• B Neyman's bias
• C Recall bias
• D Lead-time bias ✓

Explanation

Screening detected the cancer earlier, but the time of death was unchanged, so apparent survival increased only because diagnosis was moved forward, not because life was prolonged. This is lead-time bias. Length bias refers to over-representation of slowly progressing indolent tumours in screened populations, which is a separate phenomenon. The definitive way to exclude lead-time bias is to compare mortality, not survival, between screened and unscreened groups.

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.