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.


METHODS
In a prospective, observational study, an HMO that formerly dispensed only Coumadin added a generic warfarin preparation (Barr Laboratories, Pomona, NY) to its formulary.


METHODS
An anticoagulation service (ACS) affiliated with an HMO that was based in St. Louis, MO.


METHODS
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.


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.
Milligan
Paul E
PE
Division of General Medical Sciences, Barnes-Jewish Hospital Blood Thinner Clinic, School of Medicine, Washington University, 660 S. Euclid Avenue, St. Louis, MO 63110-1093, USA.
Banet
Gerald A
GA
Waterman
Amy D
AD
Gatchel
Susan K
SK
Gage
Brian F
BF
D000368:Aged; D000369:Aged, 80 and over; D000925:Anticoagulants / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*; D001281:Atrial Fibrillation / Q000188:drug therapy / Q000453:epidemiology; D015331:Cohort Studies; D016568:Drugs, Generic / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*; D005260:Female; D006279:Health Maintenance Organizations / Q000706:statistics & numerical data*; D006470:Hemorrhage / Q000139:chemically induced; D006801:Humans; D019934:International Normalized Ratio / Q000706:statistics & numerical data; D008297:Male; D013927:Thrombosis / Q000139:chemically induced; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*
D000925:Anticoagulants; D016568:Drugs, Generic; D014859:Warfarin