Management of Subtherapeutic INR 1.4 on Warfarin
For a patient with INR 1.4 on warfarin, increase the weekly warfarin dose by 5-20% and recheck the INR within 3-7 days, as this subtherapeutic level provides inadequate anticoagulation and requires dose adjustment without any need for vitamin K administration. 1, 2, 3
Immediate Assessment
Verify the indication for anticoagulation and confirm the target INR range (typically 2.0-3.0 for most indications including atrial fibrillation, venous thromboembolism, and bileaflet mechanical aortic valves; 2.5-3.5 for mechanical mitral valves or older valve types) 1, 2, 4
Assess thromboembolic risk during this period of subtherapeutic anticoagulation, particularly in high-risk patients such as those with mechanical mitral valves, recent thromboembolism, or history of stroke 1
Identify causes of the low INR including:
- Medication non-adherence or missed doses 2
- Recent initiation of interacting medications (particularly enzyme inducers like rifampin, carbamazepine, or phenytoin) 2
- Increased dietary vitamin K intake 1, 2
- Malabsorption states or diarrhea 5
- Genetic factors (CYP2C9 or VKORC1 variants causing warfarin resistance) 2
Dose Adjustment Strategy
Increase the total weekly warfarin dose by 10-20% for an INR of 1.4, as this represents a significantly subtherapeutic level 3
Do not use loading doses, as these increase hemorrhagic complications without providing more rapid protection against thrombosis 2, 3
Recheck INR in 3-7 days after dose adjustment, as the anticoagulant effect of warfarin is delayed and takes several days to reach steady state 2, 3
Monitoring Frequency
Test INR 2-4 times per week immediately after dose adjustments until the INR stabilizes within therapeutic range 2, 3
Once stable, gradually lengthen the interval between INR tests to a maximum of 4-6 weeks, though 1-4 weeks is typical for most patients 2
Increase monitoring frequency when other medications are initiated, discontinued, or taken irregularly, as drug interactions are a major cause of INR instability 2
Special Considerations
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) during the period of subtherapeutic INR 1
Evaluate for warfarin resistance if large daily doses (>10 mg) are required to maintain therapeutic INR, though this is rare 2
Screen for medication interactions including over-the-counter drugs, antibiotics, NSAIDs, and herbal supplements that may be affecting warfarin metabolism 1, 2
Assess dietary vitamin K intake consistency, as fluctuations in vitamin K consumption are a common cause of INR instability 1, 2
Common Pitfalls to Avoid
Do not give vitamin K for subtherapeutic INR, as vitamin K is only indicated for elevated INR values (>5.0) with or without bleeding 1, 6
Avoid making dose adjustments based on a single slightly out-of-range INR unless it is significantly subtherapeutic (as in this case with INR 1.4) or the patient is at high thromboembolic risk 3
Do not assume the patient is taking warfarin as prescribed without directly asking about adherence and reviewing the medication regimen 2
Recognize that elderly patients and those with genetic variants may require lower maintenance doses (2-4 mg daily), but an INR of 1.4 still indicates inadequate anticoagulation regardless of age 2, 3