A new screening programme for prostate cancer is introduced in city X. Five years later, men diagnosed through screening show markedly longer survival from diagnosis than men diagnosed symptomatically before the programme, yet age-specific prostate cancer mortality is unchanged. The most likely explanation is:

• A Length bias, because screening preferentially detects aggressive tumours
• B Lead time bias, because diagnosis is advanced in time without delaying death ✓
• C Recall bias affecting documentation of symptom onset dates
• D Confounding by age distribution differences between the two eras

Explanation

Screening detects disease earlier in its natural history. Even if treatment adds no delay to the time of death, the interval between diagnosis and death lengthens, inflating measured survival. Mortality is the only valid endpoint for judging screening benefit precisely because it is unaffected by lead time. Length bias refers to over-detection of slow-growing indolent tumours, which is a separate screening artefact.

Reference: Gordis Epidemiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.