Warfarin Dose Adjustment for Subtherapeutic INR
Increase the weekly warfarin dose by 5-20% (approximately 3.5-4 mg daily) and recheck INR in 3-7 days. 1
Immediate Dose Adjustment Strategy
For an INR of 1.1 on 3 mg daily warfarin with target 2-3, increase the total weekly dose by 10-15%, which translates to approximately 3.5 mg daily (21 mg weekly increased to 23-24 mg weekly). 1
The current INR of 1.1 provides inadequate anticoagulation and requires prompt dose adjustment without any need for vitamin K administration. 1
Most warfarin dose changes should alter the total weekly dose by 5-20%, and this approach prevents overcorrection while achieving therapeutic anticoagulation. 2
Monitoring Schedule
Recheck INR within 3-7 days after the dose increase to assess response. 1
Continue monitoring every 3-7 days until the INR stabilizes within the therapeutic range of 2.0-3.0. 2
Once stable, gradually extend the interval between INR tests up to a maximum of 4-6 weeks. 2
Identify Contributing Factors
Screen for medication interactions including over-the-counter drugs, antibiotics, NSAIDs, and herbal supplements that may be reducing warfarin effectiveness. 1
Assess dietary vitamin K intake consistency, as increased consumption of vitamin K-rich foods (leafy greens, broccoli) is a common cause of subtherapeutic INR. 1
Verify patient adherence to the prescribed warfarin regimen, as missed doses are a frequent cause of low INR values. 1
Bridging Anticoagulation Considerations
Do not routinely bridge with low molecular weight heparin (LMWH) for most patients with subtherapeutic INR. 1
Consider bridging anticoagulation with LMWH only in very high-risk patients: mechanical mitral valve, recent thromboembolism within 3 months, or history of thromboembolism while anticoagulated. 1
For standard indications like atrial fibrillation or venous thromboembolism, simply adjust the warfarin dose and monitor closely without bridging. 1
Critical Pitfalls to Avoid
Never administer vitamin K for subtherapeutic INR, as vitamin K is only indicated for elevated INR values with or without bleeding. 1
Avoid excessive dose increases that could lead to supratherapeutic INR within days—incremental adjustments of 10-15% are safer than doubling the dose. 2
Do not use loading doses in patients already on warfarin, as this creates risk of overanticoagulation. 2, 3