Observational cohort study of switching warfarin sodium products in a managed care organization

The effectiveness and safety of switching warfarin sodium products in a cohort of patients at a managed care organization were studied. Consecutive patients seen during routine visits at two anticoagulation clinics were requested to voluntarily use a generic warfarin sodium product (Barr Laboratories) instead of Coumadin (Dupont Pharma) for eight weeks. All patients who had received anticoagulation therapy with Coumadin for more than three months, who had a target International Normalized Ratio (INR) of 2.0-3.0 or 2.5-3.5, and who had an INR in the target range at the baseline visit were eligible for the study. The control group was composed of patients who were eligible for the study but were not asked to switch to the generic product or declined to switch products. There were 105 patients in each study group. The before-and-after differences in INR between the conversion group and the control group were small and of negligible clinical importance. Compared with the period before the product switch, the variability of the INR in patients in the conversion group was not significantly different after the switch. Of the 210 patients in the study, only 24 had a change in INR of >1.0 after the baseline visit; in each case, a factor not related to the product switch was associated with the INR change. Use of a generic warfarin sodium product (Barr Laboratories) in patients previously receiving the innovator product (Dupont Pharma) did not change the INRs more than did continued use of the innovator product by another group of patients.