Understanding and Avoiding Immortal-Time Bias in Gastrointestinal Observational Research.

Pharmacoepidemiologic analyses, in which observational data is interrogated to evaluate relationships between patterns of drug use and both beneficial and adverse outcomes, are being increasingly used in the study of inflammatory bowel disease. However, the results of many of these analyses may be corrupted by the presence of immortal person-time bias, an analytic error which can result in an overestimation of the benefits of medical therapy. In this report, we will describe immortal person-time bias, explain the mechanism through which it confers a false benefit, and guide the reader in how to identify this source of bias in the medical literature.