Evaluation of the clinical and economic impact of a brand name-to-generic warfarin sodium conversion program.
BACKGROUND
Substitution of generic warfarin initially was discouraged because of concerns regarding therapeutic failure or toxicity. Although subsequent research with AB-rated (i.e., bioequivalent) warfarin did not confirm initial concerns, the issue is not settled for all clinicians.


OBJECTIVE
We sought to provide additional information regarding the clinical and economic impact of warfarin conversion by analyzing a real-life sample of patients receiving long-term anticoagulation therapy who were switched from brand name to generic warfarin.


METHODS
Patients who had been taking warfarin for at least 180 days and had received uninterrupted oral anticoagulation 90 days before and 90 days after switching to generic warfarin were included. The switch date was based on the first time generic warfarin was dispensed from our pharmacies. The primary end point was the calculated amount of time each patient's international normalized ratio (INR) values were within the patient-specific target INR range in the 90 days before and after the switch. Data regarding adverse events and medical resource utilization were also collected. Pharmacoeconomic analyses were performed.


RESULTS
The analysis included 2299 patients. The overall difference in calculated time INR values were below (22.6% before vs 26.1% after switch, p<0.0001) and within (65.9% before vs 63.3% after switch, p=0.0002) the therapeutic INR range was statistically but not clinically significant. Only 28.0% of patients experienced a change in therapeutic INR control of 10% or less, 33.1% experienced INR control that improved by greater than 10%, and 38.9% experienced INR control that worsened by more than 10%. The difference in total treatment costs associated with brand name and generic warfarin was 3128 dollars/100 patient-years in favor of the generic product. Sensitivity analyses revealed that cost savings associated with warfarin conversion in this health care system were highly dependent on the difference between warfarin costs and cost of treating anticoagulation-related adverse events.


CONCLUSIONS
Most of these patients were successfully switched from brand name to generic warfarin. However, supplemental INR monitoring is warranted when one warfarin product is substituted for another to allow timely detection of those patients who experience significant changes in anticoagulation response.
Witt
Daniel M
DM
Clinical Pharmacy Anticoagulation Service, Kaiser Permanente Colorado Region, Westminster, USA. dan.m.witt@kp.org
Tillman
Donald J
DJ
Evans
Christy M
CM
Plotkin
Tatyana V
TV
Sadler
Melanie A
MA
D000368:Aged; D002318:Cardiovascular Diseases / Q000188:drug therapy / Q000191:economics; D003120:Colorado; D016568:Drugs, Generic / Q000009:adverse effects* / Q000191:economics; D017850:Economics, Pharmaceutical; D005260:Female; D006279:Health Maintenance Organizations; D006801:Humans; D019934:International Normalized Ratio; D008297:Male; D012189:Retrospective Studies; D014859:Warfarin
D016568:Drugs, Generic; D014859:Warfarin