A new screening test for prostate cancer detects tumours earlier, but randomized trials show no reduction in prostate cancer mortality. Patients whose cancer was detected by screening appear to survive longer after diagnosis than clinically detected cases. The most appropriate explanation for this apparent survival benefit is:

• A Lead time bias, where earlier detection increases observed survival time without delaying death ✓
• B Selection bias favouring motivated participants into screening programmes
• C Recall bias affecting documentation of date of diagnosis
• D Confounding by age differences between screened and unscreened groups

Explanation

Lead time is the interval between disease detection by screening and its usual clinical diagnosis. Screening advances the time of diagnosis, so survival measured from diagnosis appears longer even if the date of death is unchanged. This is lead time bias, and it explains why survival rates can improve without any mortality benefit. Length bias relates to overrepresentation of indolent disease in screened populations, while selection bias concerns who volunteers for screening, neither of which matches the described mechanism.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.