Substitution of Generic Warfarin for Coumadin in an HMO Setting

BACKGROUND: Substitution of generic warfarin for Coumadin presents safety concerns due to warfarin's narrow therapeutic index and because a prior generic formulation was removed from the US market after it was associated with adverse events. OBJECTIVE: To determine whether a health maintenance organization (HMO) can add generic warfarin to its formulary without adversely affecting warfarin management or increasing adverse events. DESIGN: In a prospective, observational study, an HMO that formerly dispensed only Coumadin added a generic warfarin preparation (Barr Laboratories, Pomona, NY) to its formulary. SETTING: An anticoagulation service (ACS) affiliated with an HMO that was based in St. Louis, MO. PARTICIPANTS: The cohort consisted of 182 enrollees in the ACS as of May 1, 1999. At the start of the study, these participants were taking Coumadin; by October 31, 2000, all had switched to Barr warfarin. MEASUREMENTS AND MAIN RESULTS: We collected data 8 months prior to and 10 months after the introduction of generic warfarin for the following endpoints: international normalized ratio (INR) control, frequency of INR monitoring, number of dose changes, and rate of thrombotic and hemorrhagic events. Statistical process control charts were used to differentiate between random variation in the endpoints and changes due to different warfarin formulations, and we used the Wilcoxon signed-rank test to look for a change in any endpoint after patients changed to generic warfarin. No significant differences were found in any endpoint. CONCLUSIONS: Substitution of Barr warfarin for Coumadin did not significantly affect INR control, warfarin management, or adverse events. Our findings suggest that HMOs can safely substitute at least 1 generic formulation of warfarin without extra monitoring.