A breast cancer screening programme using mammography reports a lead time of 3 years. If a woman diagnosed by screening has 10-year survival of 80% and a woman diagnosed clinically has 7-year survival of 60%, which statement correctly interprets the 'lead time bias'?
- A The apparent survival advantage partly reflects earlier timing of diagnosis without necessarily postponing death — the 3-year lead time inflates survival statistics artificially ✓
- B Screening has genuinely prolonged survival by 3 years
- C The 60% vs 80% difference confirms screening reduces case-fatality by 20%
- D Lead time bias causes overestimation of disease incidence in the screened group
Correct answer: A. The apparent survival advantage partly reflects earlier timing of diagnosis without necessarily postponing death — the 3-year lead time inflates survival statistics artificially
Explanation
Lead time bias occurs when screening advances the time of diagnosis without altering the natural course of disease; survival measured from diagnosis (rather than from death) is artificially extended by the lead time, making screened patients appear to live longer even if their actual date of death is unchanged. To determine true benefit, studies must compare age-specific mortality rates (not survival from diagnosis) between screened and unscreened populations. A 3-year lead time means up to 3 years of apparent 'extra survival' may be artefactual.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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