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Letter
Immortal time bias in estimates of mortality among infliximab-treated patients with Crohn's disease
  1. James D Lewis
  1. Correspondence to Dr J D Lewis, Center for Clinical Epidemiology and Biostatistics, Department of Medicine, Division of Gastroenterology, University of Pennsylvania, USA; lewisjd{at}mail.med.upenn.edu

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I read with interest the recent article by Fidder and colleagues on the long-term safety of infliximab.1 The study addresses questions of high interest to our field. However, the design of the study may have led to an under-estimate of the relative risk of cancer and death among patients treated with infliximab. Specifically, the study design may have resulted in immortal time bias.2

Immortal time bias results from differential inclusion or exclusion between the study groups of follow-up time where the outcome of interest could not have occurred. In the study by Fidder et al, follow-up time for patients who were never treated with infliximab started in 1994 or the time that the patient was diagnosed with …

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