What should I do for a patient on Coumadin (warfarin) with a subtherapeutic International Normalized Ratio (INR) of 1.9, currently taking 5 mg alternating with 2.5 mg?

Management of Subtherapeutic INR on Warfarin

Increase your patient's weekly warfarin dose by 10-20% and recheck the INR in 3-5 days. 1, 2

Immediate Dose Adjustment

Your patient is currently taking approximately 3.75 mg daily (alternating 5 mg and 2.5 mg), which equals 26.25 mg per week. With an INR of 1.9, she is slightly below the therapeutic range of 2.0-3.0 and at increased risk of thromboembolism. 1, 2

Recommended dosing strategy:

• Increase the weekly total by 10-15% (approximately 2.5-4 mg more per week) 1, 2
• This translates to taking 5 mg daily for 5 days and 2.5 mg for 2 days per week, or simply 5 mg daily for the entire week initially 2
• Most patients are satisfactorily maintained on 2-10 mg daily, and your patient is well within this range 2

Monitoring Schedule

Recheck the INR in 3-5 days after the dose adjustment. 1, 3 The anticoagulant effect takes 2-7 days to manifest fully after a dose change, so checking earlier than 3 days will not capture the full effect. 1

• Once the INR reaches 2.0-3.0 and remains stable for 2 consecutive measurements, you can extend monitoring to 2-3 times weekly for 1-2 weeks 1
• After sustained stability, gradually extend intervals up to 4 weeks maximum 1, 2

Critical Risk Assessment

The risk of thromboembolism increases sharply when INR falls below 2.0. 1 While an INR of 1.9 represents only mild subtherapeutic anticoagulation, studies demonstrate that thromboembolic events cluster when patients fall below this threshold. 1

Key considerations:

• The safety and effectiveness of warfarin depends critically on maintaining INR within therapeutic range 1
• An INR of 1.9 provides some anticoagulant effect but suboptimal protection 4
• Do not use loading doses, as they increase hemorrhagic complications without providing faster protection 2, 3

Investigate Contributing Factors

Before making dose adjustments, screen for reversible causes of INR instability:

Medication interactions: 2

• New medications (antibiotics, NSAIDs, acetaminophen >2g/day) can significantly alter INR 2, 5
• Herbal supplements and over-the-counter medications 2

Dietary changes: 2

• Increased vitamin K intake (leafy greens, certain oils) antagonizes warfarin 2
• Decreased oral intake or diarrhea can paradoxically elevate INR by reducing vitamin K absorption 6

Compliance issues: 1

• Missed doses are common and often unreported 1
• Consider gastroparesis in patients with unexplained INR fluctuations despite reported compliance 7

Physiologic factors: 2

• Hypothyroidism, edema, nephrotic syndrome, and hyperlipidemia can decrease warfarin response 2

Common Pitfalls to Avoid

Do not make large dose adjustments (>20% weekly change) for a single mildly subtherapeutic INR. 1, 3 Small, incremental changes of 5-20% of the weekly dose are safer and more predictable. 3

Do not recheck the INR within 24-48 hours of a dose change. 1 The full anticoagulant effect requires 2-7 days to develop, and premature testing leads to inappropriate dose escalation. 1

Avoid the temptation to give "catch-up" doses. 2 The anticoagulant effect of warfarin persists beyond 24 hours, and doubling doses increases bleeding risk without improving therapeutic control. 2

Elderly and High-Risk Patients

If your patient is elderly (>65 years) or has risk factors for bleeding, consider a more conservative 5-10% dose increase rather than 10-20%. 2 Lower maintenance doses are recommended for elderly and debilitated patients who may exhibit greater than expected PT/INR responses. 2