Difficulties in anticoagulation management during coadministration of warfarin and rifampin

The clinical significance of rifampin's, induction of warfarin metabolism i s well documented, but no published studies or case reports have quantified this interaction with respect to the international normalized ratio (INR). A patient receiving concomitant rifampin and warfarin to treat a mycobacte rial infection and intraventricular thrombus, respectively, underwent routi ne INR testing at a pharmacist-managed anticoagulation clinic to assess his anticoagulation regimen. A 233% increase in warfarin dosage over 4 months proved insufficient to attain a therapeutic INR during long-term rifampin t herapy. More aggressive titration of the warfarin dosage was needed. In add ition, a gradual 70% reduction in warfarin dosage over 4-5 weeks was necess ary to maintain a therapeutic INR after rifampin discontinuation, demonstra ting the clinically significant offset of, this drug interaction. Extensive changes in warfarin dosage are required to attain and maintain a therapeut ic INR during the initiation, maintenance, and discontinuation of rifampin.