Management of Subtherapeutic INR (1.36) in a Patient on Warfarin
For a patient on warfarin with INR 1.36, increase the weekly warfarin dose by 10-20% and recheck INR within 3-7 days. 1
Understanding the Clinical Significance
An INR of 1.36 is significantly subtherapeutic for all standard warfarin indications, which require a target INR of 2.0-3.0 for conditions like atrial fibrillation, DVT/PE, and most mechanical heart valves. 1, 2 This level provides inadequate anticoagulation and places the patient at increased risk for thromboembolic events. 1
The PT of 18.40 seconds correlates with this subtherapeutic INR, though the INR is the preferred monitoring parameter as it standardizes results across different laboratory reagents and provides more accurate assessment of anticoagulant effect. 3
Immediate Management Steps
Dose Adjustment Algorithm
Increase the total weekly warfarin dose by 10-20% rather than making dramatic daily dose changes, as this approach provides more stable anticoagulation. 1
Recheck INR within 3-7 days after dose adjustment to assess response, as the anticoagulant effect of warfarin persists beyond 24 hours and steady-state takes several days to achieve. 3, 2
Monitor INR 2-4 times per week during this adjustment period until the therapeutic range is reached and sustained for 2 consecutive days. 3, 4
Critical Considerations Based on Indication
The urgency and approach differ significantly based on why the patient is anticoagulated:
For DVT/PE patients: Inadequate anticoagulation at INR 1.36 significantly increases risk of thrombus extension and recurrent thromboembolism. 1 Consider bridging therapy with enoxaparin 1 mg/kg subcutaneously twice daily (or 1.5 mg/kg once daily) until INR reaches therapeutic range if the patient has acute VTE or very high thrombotic risk. 1
For atrial fibrillation: Maximum stroke protection requires INR >2.0, though some benefit remains at INR 1.5-1.9. 5 At 1.36, protection is minimal and dose adjustment is urgent. 5
For mechanical heart valves: This is a particularly high-risk situation requiring immediate attention, as valve thrombosis risk is substantial with subtherapeutic anticoagulation. 2, 5
Investigation of Underlying Causes
Before simply increasing the dose, identify and correct factors that may have caused the INR drop:
Recent medication changes or additions that may induce warfarin metabolism (e.g., rifampin, carbamazepine, phenytoin). 2
Dietary changes with increased vitamin K intake (leafy green vegetables, certain nutritional supplements). 3, 2
Poor medication compliance or missed doses, which is common and should be addressed directly with the patient. 3
Intercurrent illness affecting absorption or metabolism. 3
Drug-drug interactions from newly started medications. 2
Monitoring Protocol Going Forward
Once therapeutic INR is achieved and sustained for 2 consecutive days, reduce monitoring frequency to 2-3 times weekly for 1-2 weeks. 3, 2
After stability is demonstrated, monitoring can be reduced to weekly for 1 month, then every 2-4 weeks once the patient maintains consistent therapeutic INR values. 3, 2
Resume frequent monitoring (2-4 times weekly) whenever dose adjustments are required or when other medications are initiated, discontinued, or taken irregularly. 3, 2
Common Pitfalls to Avoid
Do not give a "loading dose" to rapidly increase INR, as this increases hemorrhagic complications without providing more rapid protection against thrombi formation. 2, 4
Do not make dose adjustments based on a single slightly out-of-range INR once the patient is stable, but at 1.36 (significantly subtherapeutic), immediate action is required. 4
Do not overlook the need for bridging therapy in very high-risk patients (acute DVT/PE, mechanical valves, recent stroke) where even brief periods of inadequate anticoagulation carry substantial morbidity risk. 1
Avoid making daily dose changes without considering the total weekly dose, as warfarin has a long half-life and effects accumulate over days. 4