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The influence of fibrate initiation on INR and warfarin dose in patients receiving chronic warfarin therapy

Journal of Thrombosis and Thrombolysis
John F. Polnak, Thomas Delate, Nathan P. Clark


Several drug interaction compendia report a risk of warfarin potentiation after initiation of a fibrate; however, the evidence of this interaction is limited. The objective of this study was to evaluate warfarin dose and international normalized ratio (INR) response among a large sample of patients receiving chronic warfarin who initiated a fibrate. This was a retrospective, one-sample, pre-to-post study. Adult patients who were receiving chronic warfarin therapy at the time of gemfibrozil or fenofibrate dispensing between 1/1/2000 and 3/31/2016 were included. Patients had at least one and two therapeutic INRs during the 90 days prior to (baseline) and after (follow-up), respectively, fibrate initiation. Comparison of stable warfarin dose:INR ratio between the baseline and follow-up periods and assessment of safety outcomes during follow-up were performed. There were 321 patients included. Patients were predominantly male (62.6%) with an indication of atrial fibrillation (44.2%). The mean warfarin dose:INR ratio was equivalent between the baseline and follow-up periods (13.4 mg/INR [± 6.9] vs. 13.5 mg/INR [± 7.5], respectively, p = 0.711). Rates of thromboembolism, bleeding, and all-cause mortality in the 90-day follow up were 0, 0.6, and 1.2%, respectively. Although individual patients may have labile INRs after fibrate initiation, no significant interaction between fibrate and warfarin in a large sample of real world patients was identified. The utility of additional INR monitoring after fibrate initiation in otherwise stable patients receiving chronic warfarin therapy is unclear.

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