Propensity Score Weighting Compared to Matching in a Study of Dabigatran and Warfarin.

INTRODUCTION: Comparing medications in observational settings requires differences in patient characteristics to be accounted for. Propensity score (PS) methods can address these differences, but PS weighting approaches may introduce bias.

METHODS: Within a cohort study of anticoagulant initiators from October 2010 through to December 2012, PS values for dabigatran relative to warfarin were estimated, and study outcomes (stroke or major bleeding) among the cohort were identified. The PS was used to match initiators and results compared with those obtained using inverse probability of treatment weighting (IPTW) and standardized morbidity ratio (SMR) weighting. Hazard ratios (HRs) for study outcomes were estimated using a proportional hazards regression model.

RESULTS: There were 23,543 dabigatran and 50,288 warfarin initiators, and matching formed 19,189 pairs (81.5% of dabigatran initiators) which resulted in a pooled stroke HR of 0.77 (95% confidence interval [CI] 0.54-1.09), and a pooled major hemorrhage HR of 0.75 (95% CI 0.65-0.87). The IPTW results for stroke (HR = 0.00; 95% CI 0.00-0.56) and major hemorrhage (HR = 0.08; 95% CI 0.08-0.10) substantially differed, while the SMR-weighted results for stroke (HR = 0.65; 95% CI 0.42-1.03) and major hemorrhage (HR = 0.73; 95% CI 0.61-0.85) differed only slightly from matching.

CONCLUSIONS: In this example, different applications of the same PS led to substantially different results, a finding that was particularly apparent with IPTW, and this was remedied by truncating extreme weights. If IPTW is used, information regarding the weights applied along with sensitivity analyses could avoid misrepresentation of study results, and would enhance their interpretation.