A screening programme detects prostate cancer in a 70-year-old man who then lives 5 years after diagnosis and dies of an unrelated cause. His neighbour, diagnosed clinically with the same tumour at the same age without screening, also dies at 75. Critics argue the screened patient did not actually live longer; his tumour was merely detected earlier. This apparent benefit of screening illustrates which bias?
- A Lead time bias ✓
- B Length bias
- C Neyman's survival bias
- D Healthy worker effect
Correct answer: A. Lead time bias
Explanation
Lead time is the interval between disease detection by screening and its usual clinical diagnosis. Screening advances the date of diagnosis without postponing death, so survival from diagnosis looks longer even though the natural history is unchanged. Length bias, the key distractor, refers instead to screening preferentially detecting slow-growing, better-prognosis tumours because they remain in the detectable preclinical phase longer.
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.