ワルファリンとリファンピシン併用患者のカルテ調査による相互作用の検討

Abstract

Rifampicin's ability to induce hepatic-microsomal enzymes is well known. As a result, rifampicin has often been implicated in drug interactions by reducing the effectiveness of many drugs that are metabolized by the hepatic-microsomal enzyme system. One such well known interaction is that of rifampicin and warfarin. The concomitant administration of rifampicin and warfarin results in the need for an unusually high maintenance dose of warfarin in order to obtain the desired therapeutic effect.The interaction between warfarin and rifampicin was investigated. The patients treated with rifampicin were selected from the prescription database file made for the prescriptions order entry system, and which contains all the prescriptions filled for the inpatients and outpatients. Thirty patients were treated with rifampicin between July 2000 and June 2001. Four of them used warfarin concomitantly during the study period. Decreased INR values were observed in all patients between 5 and 10 days after the initiation of rifampicin therapy. To maintain proper INR, rifampicin therapy resulted in the need for a two-to three-fold increase in the warfarin dosage. Subsequently, low INR/Dose values were observed during the concomitant use of warfarin and rifampicin (30%-50%). A decrease in the anticoagulant activity of warfarin frequently occurs when rifampicin is used concomitantly. After the initiation and discontinuation of rifampicin therapy, frequent coagulation tests are required in patients treated with warfarin. In conclusion, warfarin should be used with caution when rifampicin is used concomitantly.