What dose adjustment is needed for a patient on Coumadin (warfarin) 2.5 mg PO daily with a subtherapeutic International Normalized Ratio (INR) for chronic Deep Vein Thrombosis (DVT)?

Warfarin Dose Adjustment for Subtherapeutic INR in Chronic DVT

The warfarin dose should be increased by 20% (from 2.5 mg to 3.0 mg daily) for a patient with chronic DVT and subtherapeutic INR of 1.35. 1

Rationale for Dose Adjustment

The current INR of 1.35 is significantly below the therapeutic range of 2.0-3.0 that is recommended for patients with DVT 2, 1. Subtherapeutic anticoagulation increases the risk of:

• Recurrent venous thromboembolism
• Development of post-thrombotic syndrome (PTS) - patients with an INR below 2.0 for >20% of treatment time have 1.88 times higher risk of developing PTS 3

Dose Adjustment Protocol

1. Current situation:

   • Patient on warfarin 2.5 mg PO daily
   • INR = 1.35 (subtherapeutic)
   • Indication: Chronic DVT

2. Recommended adjustment:

   • Increase dose by 20% to 3.0 mg PO daily 4, 1
   • Recheck INR within 4-7 days after dose change 4
   • Continue to adjust until INR reaches therapeutic range (2.0-3.0)

3. Monitoring after adjustment:

   • Once therapeutic range is achieved, check INR twice weekly until stable 4
   • When stable for 2 consecutive measurements, transition to regular monitoring schedule

Important Considerations

• Target INR: The therapeutic range for DVT is 2.0-3.0 1, 5
• Parenteral anticoagulation: If the patient is currently on parenteral anticoagulation (LMWH, UFH), it should be continued until the INR is in the therapeutic range for 2 consecutive days 2
• Age factor: Younger patients (<60 years) may require higher doses to reach therapeutic INR 6
• Monitoring frequency: More frequent INR monitoring is essential during dose adjustments to prevent both thrombotic and bleeding complications 4

Potential Pitfalls to Avoid

1. Excessive dose increases: Increasing the dose too aggressively may lead to supratherapeutic INR and bleeding risk
2. Inadequate follow-up: Failing to recheck INR within 4-7 days may prolong subtherapeutic anticoagulation
3. Drug interactions: Always assess for new medications that might affect warfarin metabolism
4. Dietary changes: Significant changes in vitamin K intake can affect INR stability

Special Considerations for Chronic DVT

For patients with chronic DVT, maintaining therapeutic anticoagulation is crucial to prevent recurrence and complications. Research shows that patients with incomplete DVT resolution due to subtherapeutic INR values have significantly higher rates of chronic venous insufficiency symptoms (62% vs 11%) 7.

The dose adjustment recommended here aims to achieve the target INR of 2.0-3.0, which has been shown to minimize the rate of incomplete DVT resolution and reduce recurrence risk 7, 5.