Warfarin Dose Adjustment for Subtherapeutic INR
Increase the warfarin dose by 10-15% of the total weekly dose and give 2.5-3mg today, then continue with an adjusted maintenance dose to achieve the target INR of 2.5-3.5. 1
Dose Calculation and Rationale
For this patient with INR 1.9 (target 2.5-3.5):
- Current weekly dose: The patient has been taking reduced doses (1.5mg × 2 days, then 2mg × 1 day = 5mg over 3 days), but the usual maintenance is 2mg daily = 14mg/week
- INR 1.9 requires a 10% weekly dose increase per the European Heart Association algorithm for INR 1.6-1.9 1
- Today's dose should be 2.5-3mg to begin correcting the subtherapeutic level 1, 2
- New maintenance dose should be approximately 2.2-2.3mg daily (15.4-16.1mg weekly, representing a 10-15% increase) 1
Key Management Principles
The dose reduction over the past 3 days (from 2mg to 1.5mg) was inappropriate and caused the INR drift downward. This patient requires dose escalation, not reduction, given the target range of 2.5-3.5. 1, 2
Why This Target Range Matters
- INR target of 2.5-3.5 indicates high-risk thrombosis - likely mechanical mitral valve, mechanical aortic valve with risk factors, or history of thromboembolism on standard anticoagulation 3
- The current INR of 1.9 provides inadequate protection against thromboembolism in these high-risk patients 4
- Each day below therapeutic range increases thrombotic risk by approximately 0.03-0.05% in high-risk patients with mechanical valves 3
Monitoring Strategy
- Recheck INR in 3-5 days after dose adjustment to assess response 2, 5
- Once stable in therapeutic range, extend monitoring to weekly, then every 2-4 weeks 2
- Resume frequent monitoring (2-3 times weekly) after any dose change 2
Common Pitfalls to Avoid
Do not make excessive dose changes for single INR deviations, but this patient has a consistent downward trend over 3 days requiring intervention. 1, 2
Do not add bridging heparin for a single subtherapeutic INR in an otherwise stable patient, as this increases bleeding risk without clear benefit. 1, 2 However, if this patient has a mechanical mitral valve and INR remains below 2.0 for more than 48 hours, consider therapeutic bridging. 3
Avoid overreacting to the recent dose reduction - simply return to the previous stable dose (2mg) plus a 10-15% increase, rather than making dramatic changes that cause INR instability. 1, 2
Practical Implementation
- Give 2.5mg today (intermediate dose to begin correction)
- Resume 2.2-2.3mg daily starting tomorrow (can alternate 2mg and 2.5mg on different days to achieve weekly target)
- Investigate why doses were reduced - was there bleeding, drug interaction, or dietary change? 5
- Ensure consistent vitamin K intake, as fluctuations affect warfarin response 2
- Review for drug interactions, particularly NSAIDs, antibiotics, or new medications that may have contributed to INR changes 2, 5