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Difficulties in anticoagulation management during coadministration of warfarin and rifampin
1Division of Pharmacotherapy, School of Pharmacy, University of North Carolina at Chapel Hill, USA.
Abstract:
The clinical significance of rifampin's induction of warfarin metabolism is 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 mycobacterial infection and intraventricular thrombus, respectively, underwent routine 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 therapy More aggressive titration of the warfarin dosage was needed. In addition, a gradual 70% reduction in warfarin dosage over 4-5 weeks was necessary to maintain a therapeutic INR after rifampin discontinuation, demonstrating the clinically significant offset of this drug interaction. Extensive changes in warfarin dosage are required to attain and maintain a therapeutic INR during the initiation, maintenance, and discontinuation of rifampin.
Insights
Rifampin significantly increases warfarin dosage needs, requiring careful monitoring and adjustments. Dosage must be carefully managed when starting, continuing, or stopping rifampin to maintain therapeutic international normalized ratio (INR) levels.
Area of Science:
- Pharmacology
- Drug Interactions
- Clinical Pharmacy
Background:
- The interaction between rifampin and warfarin is known to affect drug metabolism.
- Quantifying this interaction using the international normalized ratio (INR) has not been previously documented.
Observation:
- A patient on rifampin for mycobacterial infection and warfarin for an intraventricular thrombus showed reduced warfarin efficacy.
- Warfarin dosage increased by 233% over 4 months was insufficient to achieve a therapeutic INR during rifampin therapy.
Findings:
- Significant warfarin dosage escalation is required to achieve therapeutic INR levels during rifampin co-administration.
- A 70% warfarin dosage reduction was necessary after rifampin discontinuation to maintain therapeutic INR.
Implications:
- Clinicians must anticipate and manage substantial warfarin dosage adjustments during rifampin initiation, maintenance, and cessation.
- Pharmacist-managed anticoagulation clinics are crucial for monitoring and titrating warfarin in patients receiving rifampin.