Warfarin Dose Adjustment for Subtherapeutic INR
For a patient on warfarin 6 mg daily with INR dropping from 2.2 to 1.8, an extra 2 mg dose is NOT the recommended approach—instead, increase the weekly dose by 5-20% (approximately 2-4 mg per week total) and recheck INR in 1-2 weeks. 1, 2
Why a Single Bolus Dose is Inappropriate
- A one-time 2 mg bolus will cause a transient spike in INR but will not achieve stable therapeutic anticoagulation, as warfarin dosing adjustments should modify the steady-state maintenance regimen rather than provide loading doses 2
- The American College of Chest Physicians recommends that for a single INR reading ≤0.5 below therapeutic range (your patient is 0.2-0.4 below target of 2.0-3.0), you can actually continue the current dose without any adjustment and simply retest in 1-2 weeks 1
- Evidence from 3,961 patients demonstrates that warfarin doses do not need immediate adjustment for INR values between 1.7 and 3.3, suggesting your patient's INR of 1.8 may not require intervention at all 1
Recommended Dosing Strategy
If you decide adjustment is warranted:
- Increase the total weekly dose by 5-20%, which translates to adding 2-8 mg to the weekly total (currently 42 mg/week) 2
- For practical dosing: increase from 6 mg daily to 6.5-7 mg daily (alternating 6 mg and 7 mg on different days achieves 6.5 mg average) 2
- Recheck INR in 1-2 weeks after the dose adjustment to assess response 1, 2
Critical Considerations Before Adjusting
Investigate potential causes of INR decline:
- Recent dietary changes, particularly increased vitamin K intake from green leafy vegetables 3
- New medications that induce warfarin metabolism or reduce absorption 4
- Gastrointestinal issues affecting absorption 3
- Medication non-adherence 1
When NOT to Adjust
- For a single subtherapeutic INR with previously stable values, maintaining the current dose is reasonable 1
- A retrospective study of 2,597 patients showed no significant difference in thromboembolic events between patients with subtherapeutic INR and those with therapeutic INR 1
- Even in high-risk patients with mechanical heart valves, thromboembolic event incidence was only 0.4% without intervention 1
Bridging Therapy is NOT Indicated
- Routine heparin bridging for a single subtherapeutic INR increases bleeding risk without clear benefit 1
- Bridging should only be considered for patients with mechanical heart valves or other high-risk conditions with persistently low INR on multiple readings 5, 1
Common Pitfalls to Avoid
- Never use loading doses or bolus adjustments—they cause INR instability and do not improve outcomes compared to steady-state dose modifications 2
- Avoid overreacting to single out-of-range values—immediate dose adjustments for minor INR deviations do not improve outcomes 1
- Do not make frequent small adjustments—this creates a "chasing the INR" pattern that destabilizes anticoagulation 2