What to do for a patient with a subtherapeutic International Normalized Ratio (INR) of 1.2 on warfarin therapy?

Management of INR 1.2 on Warfarin

Increase your weekly warfarin dose by 5-20% and recheck the INR within 3-7 days, as this subtherapeutic level provides inadequate anticoagulation and requires dose adjustment. 1

Immediate Actions

• Do not administer vitamin K - vitamin K is only indicated for elevated INR values (>5.0) with or without bleeding, never for subtherapeutic levels 1
• Verify the indication for anticoagulation and confirm the target INR range, which is typically 2.0-3.0 for most indications including atrial fibrillation, venous thromboembolism, and bileaflet mechanical aortic valves 1, 2
• For mechanical mitral valves or older valve types, the target range is higher at 2.5-3.5 1, 2

Dose Adjustment Strategy

• Increase the total weekly warfarin dose by 5-20% based on how far below target the INR is and the stability of previous INR values 1
• Recheck INR within 3-7 days after dose adjustment to assess response 1
• Most patients are satisfactorily maintained at 2-10 mg daily once therapeutic range is achieved 2

Identify Contributing Factors

• Screen for medication interactions including over-the-counter drugs, antibiotics, NSAIDs, and herbal supplements that may be affecting warfarin metabolism 1, 2
• Assess dietary vitamin K intake consistency - increased consumption of vitamin K-rich foods (leafy greens, certain oils) is a common cause of INR instability and can lower the INR 1, 2
• Review patient adherence to prescribed warfarin dosing 3
• Consider enzyme-inducing medications that may be accelerating warfarin metabolism 2

Assess Thromboembolic Risk

• Evaluate the patient's risk during this period of subtherapeutic anticoagulation, particularly in high-risk patients 1
• High-risk patients include those with:
  • Mechanical mitral valves 1
  • Recent thromboembolism within 3 months 1
  • History of thromboembolism while anticoagulated 1
  • History of stroke in atrial fibrillation patients 1

Bridging Anticoagulation Considerations

• Bridging with low molecular weight heparin (LMWH) should only be considered in very high-risk patients as listed above 1
• For most patients with INR 1.2, simply increasing the warfarin dose without bridging is appropriate 1
• The decision to bridge depends on balancing thromboembolic risk against bleeding risk from dual anticoagulation 1

Monitoring Schedule

• After dose adjustment, recheck INR within 3-7 days 1
• Once stable in therapeutic range, INR monitoring can occur every 1-2 months if stability is maintained 4
• More frequent monitoring is needed during fluctuations in diet, weight changes, medication changes, or intercurrent illness 4

Common Pitfalls to Avoid

• Never give vitamin K for subtherapeutic INR - this is a critical error that will further suppress anticoagulation 1
• Do not make excessive dose increases (>20% weekly dose) as this can lead to supratherapeutic INR overshoot 1
• Do not forget to investigate the cause of the low INR to prevent recurrence 1
• Avoid using insensitive thromboplastin reagents (ISI >1.5) for INR monitoring as they provide less reliable results 4