Rationale: Observational studies report a significant protective effect of antifibrotics on mortality among patients with idiopathic pulmonary fibrosis (IPF). Many of these studies, however, were subject to immortal time bias because of the mishandling of delayed antifibrotic initiation. Objectives: To evaluate the antifibrotic effect on mortality among patients with IPF using appropriate statistical methods that avoid immortal time bias. Methods: Using a large administrative database, we identified 10,289 patients with IPF, of whom 2,300 used antifibrotics. Treating delayed antifibrotic initiation as a time-dependent variable, three statistical methods were used to control baseline characteristics and avoid immortal time bias. Stratified analysis was performed for patients who initiated antifibrotics early and those who initiated treatment late. For comparison, methods that mishandle immortal time bias were performed. A simulation study was conducted to demonstrate the performance of these models in a wide range of scenarios. Results: All three statistical methods yielded nonsignificant results for the antifibrotic effect on mortality, with the stratified analysis for patients with early antifibrotic initiation suggesting evidence for reduced mortality risk (for all patients, hazard ratio, 0.89; 95% confidence interval, 0.79-1.01; P = 0.08; for patients who were 65 years or older, hazard ratio, 0.85; 95% confidence interval, 0.73-0.98; P = 0.03). Methods that mishandle immortal time bias demonstrated significantly lower mortality risk for antifibrotic users. Bias of these methods was evident in the simulation study, where appropriate methods performed well with little to no bias. Conclusions: Findings in this study did not confirm an association between antifibrotics and mortality, with a stratified analysis showing support for a potential treatment effect with early treatment initiation.
Keywords: IPF; antifibrotics; mortality; observational study.