Warfarin Dose Adjustment for Low INR on 2 mg Five Times Weekly
For a patient taking 2 mg of warfarin five times weekly (10 mg total weekly dose) with a low INR, increase the weekly dose by 10-15% depending on the specific INR value, which translates to adding approximately 1-1.5 mg to the weekly total. 1
Specific Dose Adjustment Algorithm
The adjustment depends on the actual INR value:
If INR is 1.6-1.9 (mildly subtherapeutic):
- Increase the weekly dose by 10% 1
- Current dose: 10 mg/week → New dose: 11 mg/week
- Practical implementation: Take 2 mg six days per week instead of five days, or alternate between 2 mg daily for 5 days and 3 mg for 1 day 1
If INR is <1.5 (significantly subtherapeutic):
- Increase the weekly dose by 15% 1
- Current dose: 10 mg/week → New dose: 11.5 mg/week
- Practical implementation: Take 2 mg five days per week plus 1.5 mg one day per week, or 2 mg six days per week 1
If INR is only slightly below target (≤0.5 below therapeutic range):
- Continue the current dose without adjustment if this is a single reading in a previously stable patient 2
- Recheck INR in 1-2 weeks 2
- The American College of Chest Physicians recommends against immediate dose changes for minor deviations in stable patients 2
Important Considerations for This Patient
Do not use heparin bridging for a single subtherapeutic INR. Routine bridging increases bleeding risk without clear benefit, even in high-risk patients 2. Studies show only 0.4% thromboembolic event rate in mechanical valve patients without bridging 2.
Assess for causes of low INR before adjusting:
- Medication interactions (particularly antibiotics, antifungals, NSAIDs) 1
- Changes in vitamin K intake 1
- Medication non-adherence 3
- Intercurrent illness 3
Follow-Up Monitoring
Recheck INR within 1 week after dose adjustment 1. Once the INR stabilizes in therapeutic range:
- Monitor 2-3 times weekly for 1-2 weeks 1
- Then weekly for the first month 1
- Then every 2-4 weeks once stable 1
Critical Pitfalls to Avoid
Do not make large dose adjustments (>20% weekly dose change) for minor INR deviations. Most changes should alter the total weekly dose by only 5-20% 4. Excessive adjustments lead to INR instability 4.
Do not adjust the dose for a single slightly low INR in a previously stable patient. Evidence shows that immediate dose adjustments for minor INR deviations do not improve outcomes compared to maintaining the current dose 2, 5. A randomized trial of 160 patients found no significant difference in follow-up INR control between single-dose adjustment versus no adjustment (60% vs 56% in therapeutic range) 5.
Avoid loading doses or dramatic increases. The dose-response relationship is not linear, and small percentage changes in weekly dose produce meaningful INR changes over time 4.