Warfarin Dose Adjustment During Rifampicin Therapy: A Case Report with Practical Monitoring Implications

Summary

Rifampicin markedly reduces warfarin anticoagulant effects through enzyme induction, requiring careful dose adjustment and INR monitoring. This case report describes pharmacist-guided management of a patient receiving concurrent rifampicin and warfarin therapy, highlighting practical strategies for maintaining therapeutic anticoagulation during this clinically challenging drug interaction.

Abstract

Warfarin management is challenging because its anticoagulant effect is influenced by drug-drug interactions and individual variability. Rifampicin is a potent inducer of cytochrome P450 enzymes and can substantially reduce warfarin efficacy, making structured monitoring and individualized dose adjustment necessary.

This report aimed to describe the clinical management strategy for maintaining therapeutic anticoagulation in a patient receiving concurrent warfarin and rifampicin therapy, with a focus on International Normalized Ratio (INR) monitoring, dose adjustment, and pharmacist-led management.

We present a clinical case of a 64-year-old Chinese male with chronic thromboembolic pulmonary hypertension who required long-term warfarin therapy and subsequently received rifampicin-containing antituberculosis treatment. The patient's clinical course, INR values, warfarin dose adjustments, and safety outcomes were retrospectively reviewed. A structured pharmacist-supported monitoring approach was applied during rifampicin co-administration.

After rifampicin initiation, the patient's warfarin requirement increased from 1.5 mg/day to 4.5 mg/day, representing an approximately 200% increase. INR was monitored twice weekly during the initial phase and subsequently weekly until stabilization. A therapeutic INR range (2–3) was achieved after approximately 2 months without thromboembolic events or major bleeding complications.

Rifampicin-warfarin co-administration requires individualized management rather than fixed dose adjustment strategies. Frequent INR monitoring, timely dose modification, and pharmacist involvement may facilitate safe maintenance of anticoagulation during this clinically significant drug–drug interaction.

Introduction

Warfarin is an anticoagulant drug that belongs to the class of coumarin derivatives. It works by interfering with the conversion of vitamin K to its 2,3-epoxide form (which is the vitamin K epoxide) and thereby decreasing the production of vitamin K-dependent coagulation factors (coagulation factors II, VII, IX, and X)¹. Warfarin has become one of the most frequently prescribed oral anticoagulants in clinical practice because it can be taken orally and is effective in treating thromboembolic disease (venous thromboembolism), as well as in treating patients who require heart valve replacement surgery and patients with atrial fibrillation. Due to....

Protocol

All procedures performed in this case report involving human participants were conducted in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The institutional review board waived ethical approval because this report described routine clinical care without experimental intervention. Written informed consent for publication of this case report and any accompanying data was obtained from the patient.

1. Baseline patient assessment

1. Evaluation of anticoagulation status
   ....

Results

Anticoagulation response during rifampicin therapy

Before rifampicin initiation, the patient had maintained stable anticoagulation with warfarin at 1.5 mg/day (10.5 mg/week), with a target INR of 2.0–3.0. After rifampicin-containing antituberculosis therapy was initiated, the patient's warfarin requirement gradually increased, reflecting reduced anticoagulant exposure associated with rifampicin-mediated enzyme induction.

The maintenance warfarin dos.......

Discussion

Rifampicin-warfarin interaction represents a clinically important challenge because rifampicin can substantially reduce anticoagulant efficacy through enzyme induction. Warfarin consists of two enantiomers, with S-warfarin primarily metabolized by CYP2C9 and R-warfarin metabolized mainly by CYP1A2 and CYP3A4. Rifampicin induces these metabolic pathways, resulting in increased clearance of warfarin and reduced anticoagulant activity^6,7,

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CHA2DS2-VASc score	European Society of Cardiology guideline resource	https://www.escardio.org	Assessment of thromboembolic risk in atrial fibrillation
Computed tomography scanner	China–Japan Friendship Hospital Radiology Department	Institutional equipment	Diagnosis and follow-up evaluation of pulmonary tuberculosis
Enoxaparin sodium injection	Sanofi	Product information: https://www.sanofi.com	Bridging anticoagulation during temporary warfarin interruption
GraphPad Prism	GraphPad Software LLC	https://www.graphpad.com	Figure preparation and visualization
HAS-BLED score	European Society of Cardiology guideline resource	https://www.escardio.org	Assessment of bleeding risk
International normalized ratio (INR) measurement	Laboratory Department, China–Japan Friendship Hospital	Institutional laboratory procedure	Monitoring anticoagulation intensity
Microsoft Excel	Microsoft Corporation	https://www.microsoft.com	Data recording and clinical timeline organization
Rifampicin capsules	Zhejiang Shapuaisi Pharmaceutical Co., Ltd., China	Product information available from manufacturer	Antituberculosis therapy and enzyme induction exposure
Sputum culture for Mycobacterium tuberculosis	Clinical Microbiology Laboratory, China–Japan Friendship Hospital	Institutional laboratory procedure	Confirmation of tuberculosis diagnosis
Warfarin sodium tablets	Orion Corporation, Finland	Product information: https://www.orionpharma.com	Long-term anticoagulation and dose adjustment
WHO bleeding scale	World Health Organization	>https://www.who.int	Evaluation of bleeding complications