Management of Subtherapeutic INR (1.4) on Warfarin 4mg
For a patient on warfarin 4mg with an INR of 1.4 (subtherapeutic for standard therapeutic range 2.0-3.0), increase the weekly warfarin dose by 10-20% and recheck INR within 3-7 days. 1, 2
Dose Adjustment Algorithm
Calculate the dose increase:
- Current dose: 4mg daily = 28mg weekly
- 10% increase: 28mg × 1.10 = 30.8mg weekly (increase by ~3mg/week)
- 15% increase: 28mg × 1.15 = 32.2mg weekly (increase by ~4mg/week)
- 20% increase: 28mg × 1.20 = 33.6mg weekly (increase by ~6mg/week) 2, 3
Most appropriate adjustment for INR 1.4: A 15-20% increase is reasonable given the INR is significantly below target (more than 0.5 below the lower limit of 2.0). 1, 2
Practical implementation: Increase to 4.5mg daily (31.5mg weekly, representing a 12.5% increase), or alternate 4mg and 5mg on different days to achieve approximately 32mg weekly. 3
Critical Management Points
Do NOT administer bridging anticoagulation with heparin or LMWH for a single subtherapeutic INR. The American College of Chest Physicians found no significant difference in thromboembolic events between patients with single low INRs versus therapeutic INRs, even in high-risk patients with mechanical heart valves. 1
Recheck INR timing:
- Recheck INR in 3-5 days after implementing the dose increase 2
- Continue monitoring 2-4 times weekly until INR stabilizes within therapeutic range (2.0-3.0) 2, 3
- Once stable, gradually extend monitoring intervals up to maximum 4-6 weeks 3
Investigation of Underlying Causes
Before resuming therapy, identify why INR dropped: 4
- Medication non-adherence or missed doses
- New drug interactions (antibiotics, antifungals, anticonvulsants)
- Dietary changes (increased vitamin K intake from green leafy vegetables)
- Intercurrent illness affecting absorption
- Weight changes
- Alcohol consumption patterns 4
Common Pitfalls to Avoid
Do not hold warfarin doses unnecessarily. An INR of 1.4 requires dose escalation, not withholding. Holding doses is only appropriate when INR is supratherapeutic (>5.0 without bleeding, or >3.0-5.0 depending on bleeding risk). 1, 2
Avoid excessive dose adjustments. Making changes larger than 20% of the weekly dose can lead to INR instability and overshooting the therapeutic range. 2, 3
Do not use vitamin K for subtherapeutic INR. Vitamin K is reserved for supratherapeutic INR (>5.0 without bleeding or any INR with active bleeding), not for low INR values. 5, 2
Therapeutic Target Confirmation
Verify the appropriate INR target range for this patient's indication: 6, 7
- Standard intensity (2.0-3.0): Most indications including atrial fibrillation, venous thromboembolism, bileaflet mechanical aortic valves 6, 7
- Higher intensity (2.5-3.5): Certain mechanical heart valves (caged ball, caged disc), recurrent systemic embolism 6, 7
The laboratory report indicates a standard therapeutic range of 2.0-3.0, confirming that 1.4 is indeed subtherapeutic. 6