Lead-time bias in cancer screening occurs when:

• A Screening preferentially detects slow-growing tumours with better prognosis (length-biased sampling)
• B Survival time appears longer after screening diagnosis simply because detection occurred earlier in the disease course, not because of true life extension ✓
• C Patients in screened groups are healthier volunteers with intrinsically better outcomes
• D The screening test classifies patients incorrectly due to disease spectrum variation

Explanation

Lead-time bias occurs in survival studies when early detection via screening advances the diagnosis date earlier in the disease course. If the natural history is unchanged, survival appears longer (from diagnosis to death) in the screened group simply because the clock started earlier — not because screening actually extended life. True benefit requires a reduction in disease-specific mortality, not just longer apparent survival from diagnosis. Length-biased sampling (option A) is a separate bias where slowly progressing tumours are over-represented in screening. Healthy-volunteer effect is selection 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.