A city introduces a screening programme for prostate cancer. Five years later, the median survival from date of diagnosis has risen from 3 years to 6 years, but age-specific prostate cancer mortality in the city is unchanged. The apparent improvement in survival is best explained by:

• A Lead time bias ✓
• B Length bias
• C Selection bias favouring motivated participants
• D Recall bias in exposure ascertainment

Explanation

Screening detects disease earlier in its natural history, so survival measured from diagnosis increases even when the time of death is unchanged. This is lead time bias, and it is why mortality, not survival, must be the endpoint in evaluating a screening programme. Length bias would apply if screening were selectively picking up indolent tumours, which is a different mechanism and is tested elsewhere.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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