How can case ascertainment bias arise in a cohort, and how can it be mitigated?

• A. If outcome detection differs by exposure, bias; mitigate with standardized outcome definitions and blinded adjudication
• B. If exposure misclassification occurs
• C. If loss to follow-up is high
• D. If sample size is small

Answer: (A)

Case ascertainment bias happens when the way we detect whether the outcome occurred depends on the participant’s exposure. If exposed individuals are monitored more closely, undergo more tests, or outcomes are defined or recorded differently for them, you’ll detect more outcomes in the exposed group even if the true risk is the same. That’s why this bias inflates or deflates the apparent association between exposure and outcome.

Mitigation focuses on making outcome detection uniform and objective. Use standardized, pre-specified outcome definitions applied to everyone, and have outcome adjudication done by blinded reviewers who don’t know the exposure status. Centralized or automated outcome assessment and ensuring equal follow-up intensity across groups also help prevent differential detection.

Exposure misclassification is a different issue (bias about the exposure itself), loss to follow-up relates to attrition or selection bias, and small sample size affects precision rather than how outcomes are detected.