A screening programme is introduced for a slow-progressing cancer with a long pre-clinical detectable phase (PCDP). Comparing screened cases with unscreened cases, survival appears longer in the screened group even though earlier diagnosis did not change the time of death. This represents:

• A Length-biased sampling
• B Volunteer bias
• C Over-diagnosis bias
• D Lead-time bias ✓

Explanation

Lead-time bias occurs when early detection via screening merely advances the date of diagnosis without altering the biological endpoint (death). The screened individual appears to survive longer because their diagnosis was moved earlier along the disease timeline, but actual life-span is unchanged. Length-biased sampling refers to the tendency of screening to preferentially detect slow-progressing (less aggressive) tumors. Over-diagnosis refers to detecting tumors that would never have become clinically apparent in the patient's lifetime.

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.