A screening programme detects breast cancer in women at age 52 years. Women whose cancer was detected clinically presented at age 58 years on average, yet both groups died at age 64 years. Screening appears to prolong survival by 6 years even though death was not delayed. This artifact is called:
- A Lead time bias ✓
- B Length-time bias
- C Recall bias
- D Surveillance bias
Correct answer: A. Lead time bias
Explanation
Lead time is the interval between detection by screening and usual clinical diagnosis. If screening advances diagnosis without delaying death, survival measured from diagnosis looks longer although the natural history is unchanged. Length-time bias is the separate problem that screening preferentially detects slow-growing, better-prognosis tumours. The numbers here show identical age at death, so true benefit is absent and only lead time explains the apparent gain.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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