What adjustment should be made to a patient's warfarin (coumarin) dose, currently 7.5mg on Mondays, Saturdays, and Sundays and 4mg on Tuesdays, Thursdays, and Fridays, with an International Normalized Ratio (INR) of 1.91?

Warfarin Dose Adjustment for Subtherapeutic INR

For a patient with an INR of 1.91 on a variable weekly warfarin regimen (7.5mg Monday/Saturday/Sunday and 4mg Tuesday/Thursday/Friday), the most appropriate management is to increase the total weekly warfarin dose by 10-20% and retest the INR in 1-2 weeks.

Assessment of Current Situation

The patient's current INR of 1.91 is slightly below the standard therapeutic range of 2.0-3.0 that is targeted for most indications requiring warfarin therapy 1. This represents a subtherapeutic level of anticoagulation that requires adjustment.

Current weekly dose calculation:

• 7.5mg × 3 days (Monday, Saturday, Sunday) = 22.5mg
• 4mg × 3 days (Tuesday, Thursday, Friday) = 12mg
• Total weekly dose = 34.5mg

Recommended Dose Adjustment

Since the INR is only slightly below the therapeutic range (1.91 vs. target minimum of 2.0):

1. Increase the total weekly dose by 10-20% 1

   • 10% increase = 38mg weekly (rounded)
   • 20% increase = 41.4mg weekly (rounded to 41mg)

2. This increase can be implemented by:

   • Increasing the 7.5mg doses to 8.5mg on Monday/Saturday/Sunday, OR
   • Increasing the 4mg doses to 5mg on Tuesday/Thursday/Friday, OR
   • Adding 1mg to each daily dose

3. Retest the INR in 1-2 weeks to assess the effect of the dose adjustment 2

Rationale for Recommendation

The American College of Chest Physicians (ACCP) guidelines recommend that for INR values below the therapeutic range, a 10-20% increase in the total weekly dose is appropriate 2. This modest adjustment is preferred over large dose changes that can lead to INR instability.

For patients with a single out-of-range INR that is only slightly outside the therapeutic range, guidelines suggest continuing the current dose without adjustment and retesting in 1-2 weeks 2, 1. However, since this patient's INR is below the therapeutic range (which increases thrombotic risk), a modest dose increase is warranted rather than simply monitoring.

Important Considerations

• The full impact of the adjusted anticoagulant dose may not be evident until days 3-5 after the change 1
• Making frequent or large dose changes for INRs only slightly out of range can lead to INR instability 1
• Bridging with heparin is not recommended for patients with a single subtherapeutic INR value 2
• Computer-guided warfarin dose adjustment has been shown to be superior to traditional dose regulation, particularly when personnel are inexperienced 2

Monitoring Plan

• Recheck INR in 1-2 weeks after implementing the dose change 2
• If the INR remains subtherapeutic after the initial adjustment, consider an additional 10-20% increase
• Once the INR is within the therapeutic range on two consecutive measurements, monitoring can be extended to longer intervals (up to 4-6 weeks maximum) 3

Pitfalls to Avoid

• Avoid large dose increases that may lead to excessive anticoagulation and increased bleeding risk
• Do not administer bridging therapy with heparin for a single subtherapeutic INR 2
• Do not make multiple dose adjustments before assessing the effect of the initial change
• Remember that warfarin has a narrow therapeutic window, and individual variation in response is common 4