Why is loss to follow-up a potential source of bias in cohort studies?

• A. If losses are related to exposure and outcome, estimates may be biased; conduct tracing, compare characteristics, and use methods like inverse probability weighting.
• B. Loss to follow-up is never a source of bias.
• C. Loss to follow-up improves study validity.
• D. Loss to follow-up only affects assays.

Answer: (A)

Loss to follow-up can bias results in a cohort study when the likelihood of remaining in the study is related to both the exposure and the outcome. If participants with a certain exposure are more likely to drop out, and those who drop out also have a different risk of the outcome, the group that remains is not representative of the original cohort. This creates informative censoring, distorting the estimated association between exposure and outcome. To address this, researchers may trace participants to minimize missing data, compare characteristics between those retained and those lost to see if dropout is related to key factors, and use methods like inverse probability weighting or multiple imputation to adjust for missing information. The other ideas don’t fit because loss to follow-up can indeed introduce bias, it does not improve study validity, and it isn’t limited to affecting laboratory assays.