To achieve target international normalized ratio with concurrent warfarin and rifampicin therapy is a challenge: a case report and review of literature
Keywords:Warfarin, Rifampicin, Deep vein thrombosis, Drug interaction
A 60-year-old man with a history of deep vein thrombosis put on anticoagulation therapy with warfarin 2 mg daily. Achieving a therapeutic level of anticoagulation was difficult despite escalating doses of warfarin, because of the interaction with rifampicin. A 5 to 6 fold increase in warfarin dose was prescribed to reach therapeutic international normalized ratios (INRs), but even these increases were insufficient to maintain his INR in the therapeutic range. After rifampicin was discontinued, warfarin doses were gradually reduced over the next 2 months. When concurrent warfarin-rifampicin therapy is necessary, vigilant monitoring of INR is imperative and rifampicin should be stopped. Warfarin is an oral anticoagulant used to get target INR to prevent thrombosis in various cardiovascular diseases. Its metabolism is affected by dugs, diet and individual characteristics. It is metabolized in liver by microsomal cytochrome P450 enzyme. Rifampicin is an essential component of first line antitubercular regimen. It induces enzyme P450, responsible for metabolism of warfarin. So to get target INR 2.5 to 3.5 is very difficult even with maximum possible dose of warfarin when patient taking simultaneously both drugs. In this case, rifampicin was stopped to achieve target INR. Tuberculosis patient on warfarin should not take rifampicin as component of first line antitubercular regimen.
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