A cross-sectional survey finds a strong association between regular aspirin use and reduced prevalence of colorectal polyps. A cardiologist notes that patients on aspirin are more frequently screened via colonoscopy. This phenomenon—where treatment-associated surveillance detects more disease—best exemplifies:

• A Protopathic bias
• B Length-time bias
• C Hawthorne effect
• D Detection bias (surveillance bias) ✓

Explanation

Detection bias (surveillance bias) occurs when one group is monitored more intensively than another, leading to differential diagnosis rates regardless of true disease frequency. Here, aspirin users receive more colonoscopies, so paradoxically the apparent protective effect of aspirin could be masked or a protective effect exaggerated depending on the direction. Protopathic bias occurs when a drug is prescribed for an early symptom of the disease being studied. Length-time bias affects screening studies where slow-growing tumors are over-detected. The Hawthorne effect is a behavioural change from being observed.

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

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