What to do with a warfarin (anticoagulant) dose when the International Normalized Ratio (INR) is subtherapeutic at 1.8?

Management of Subtherapeutic INR of 1.8 on Warfarin

For a patient with a subtherapeutic INR of 1.8 on warfarin therapy, the current dose should be continued and the INR should be rechecked within 1-2 weeks. 1

Assessment of Subtherapeutic INR

• An INR of 1.8 is considered subtherapeutic for most indications requiring warfarin therapy, as the typical therapeutic range is 2.0-3.0 1
• For patients with previously stable therapeutic INRs who present with a single out-of-range INR of 0.5 below therapeutic range, continuing the current dose is recommended 1
• This approach is supported by evidence showing that dose adjustments for minor deviations from therapeutic range often lead to INR overcorrection and subsequent instability 1

Management Recommendations

• Continue the current warfarin dose without adjustment 1
• Schedule INR testing within 1-2 weeks to ensure the INR returns to therapeutic range 1
• Avoid routine bridging with heparin for patients with a single subtherapeutic INR value 1
• Investigate potential causes of the INR deviation, such as:
  • Medication changes or interactions 1, 2
  • Dietary changes affecting vitamin K intake 1
  • Missed doses or compliance issues 3

Rationale for Conservative Management

• Evidence from observational studies shows no significant increase in thromboembolic events with a single subtherapeutic INR 1
• Even in patients with mechanical heart valves, the incidence of thromboembolic events was found to be only 0.3-0.4% with a single subtherapeutic INR without bridging therapy 1
• Unnecessary dose adjustments can lead to INR instability and potentially increase the risk of both bleeding and thrombotic complications 1, 4

Special Considerations

• If the patient has missed a dose, they should take it as soon as possible on the same day but should not double the dose to make up for missed doses 3
• For patients at exceptionally high risk of thromboembolism (e.g., recent thrombosis, certain mechanical heart valves), individual risk assessment may be warranted 1
• If the INR remains subtherapeutic on follow-up testing, a modest dose increase (5-20% of weekly dose) may be appropriate 4

Common Pitfalls to Avoid

• Unnecessary dose adjustments for minor INR deviations can lead to INR instability and "chasing the INR" 1, 4
• Routine bridging with heparin for a single subtherapeutic INR increases bleeding risk without clear benefit 1
• Failure to identify and address underlying causes of INR fluctuation (medication changes, dietary changes, missed doses) 2
• Inadequate follow-up monitoring, which may delay recognition of persistent subtherapeutic anticoagulation 2

Follow-up Recommendations

• Recheck INR within 1-2 weeks 1
• If INR remains subtherapeutic on follow-up, consider a modest dose increase of 5-20% of the weekly total dose 4
• More frequent monitoring is indicated until stability is re-established 2
• Reinforce medication adherence and consistency in vitamin K intake 1