What is the next step for a patient with an International Normalized Ratio (INR) of 1.7 who is taking warfarin?

Management of a Patient with INR of 1.7 on Warfarin

For a patient with an INR of 1.7 who is taking warfarin, the next step is to increase the warfarin dose to achieve the target therapeutic INR range, as this subtherapeutic level increases thrombotic risk. 1

Assessment of Current Anticoagulation Status

• An INR of 1.7 is subtherapeutic for patients on warfarin therapy, regardless of indication
• Standard target INR ranges based on clinical indication:
  • Mechanical aortic valves (bileaflet and Medtronic Hall): INR 2.0-3.0 1
  • Mechanical mitral valves (all types): INR 2.5-3.5 1
  • Other disc valves and Starr-Edwards valves: INR 2.5-3.5 1

Management Algorithm

1. Increase warfarin dose:

   • Adjust the total weekly dose by 5-20% based on the current INR value 2
   • For an INR of 1.7, which is only slightly below the therapeutic range, a 5-10% increase in weekly dose would be appropriate

2. Increase monitoring frequency:

   • Monitor INR more frequently (e.g., 2-4 times per week) until the therapeutic range is achieved 2
   • Once stable in therapeutic range, monitoring can be gradually extended to every 4-6 weeks

3. Consider bridging therapy for high-risk patients:

   • For patients at high risk of thrombosis (mechanical valves, recent thromboembolism, atrial fibrillation with additional risk factors), consider therapeutic doses of unfractionated heparin (UFH) or low molecular weight heparin (LMWH) during the period of subtherapeutic INR 3
   • Therapeutic doses of subcutaneous UFH (15,000 U every 12 h) or LMWH (100 U per kg every 12 h) may be used until therapeutic INR is achieved 3

Special Considerations

• Avoid vitamin K administration: Unlike management of supratherapeutic INR, vitamin K should not be given for subtherapeutic INR as it would further reduce anticoagulation effect

• Assess for potential causes of subtherapeutic INR:

  • Missed doses or non-adherence
  • Drug interactions (medications that induce warfarin metabolism)
  • Dietary changes (increased vitamin K intake)
  • Changes in health status (improved liver function, resolved heart failure)

• Risk assessment: The risk of thromboembolism when a patient is not adequately anticoagulated can be 10-20% per year in high-risk scenarios (e.g., mechanical prosthesis with previous thromboemboli) 3

Monitoring After Dose Adjustment

• Recheck INR within 3-5 days after dose adjustment
• Continue dose adjustments until target INR is achieved
• Once stable, gradually extend the interval between INR tests to a maximum of 4-6 weeks 2

Potential Pitfalls

• Avoid excessive dose increases: Large increases in warfarin dose can lead to overcorrection and excessive anticoagulation
• Don't ignore drug interactions: Many medications can affect warfarin metabolism and efficacy
• Consider patient-specific factors: Age, sex, comorbidities, and concomitant medications can all affect warfarin dosing requirements 4, 5
• Avoid loading doses in patients already on warfarin therapy, as this can lead to excessive anticoagulation 2

Machine learning approaches and warfarin dosing calculators may help guide individualized dosing adjustments, potentially improving time to therapeutic INR and reducing excessive anticoagulation events 5, 6.