Lead-time bias in cancer screening studies artificially inflates survival time because:

• A Patients diagnosed by screening tend to be healthier volunteers (healthy volunteer effect)
• B Survival is measured from the time of diagnosis, which is earlier with screening, without necessarily extending life ✓
• C Slow-growing tumours detected by screening have inherently better prognosis (length-time bias)
• D Screened patients receive more aggressive treatment than unscreened patients

Explanation

Lead-time bias occurs when survival (measured from diagnosis to death) appears longer in screened patients simply because the diagnosis was made earlier — not because the natural history of the disease changed or life was actually extended. The patient still dies at the same time; the survival clock just started earlier. This is a critical limitation of single-arm screening studies that report 5-year survival without a concurrent unscreened control group. Length-time bias (option C) is a separate phenomenon where screening preferentially detects slow-growing indolent tumours with better prognosis, not faster-growing lethal ones.

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

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Written and medically reviewed by the StethoPrep medical team.