Management of Subtherapeutic INR on Warfarin
For patients with stable therapeutic INRs presenting with a single subtherapeutic INR value, continue the current warfarin dose and retest the INR within 1-2 weeks without administering bridging therapy with heparin. 1
Assessment of Subtherapeutic INR
- For patients on warfarin with a subtherapeutic INR, first determine if this is a single isolated event or a persistent pattern 1
- Evaluate for potential causes of the subtherapeutic INR, including medication changes, dietary changes (especially increased vitamin K intake), poor adherence, or changes in health status 2
- Consider the patient's indication for anticoagulation and their risk for thromboembolism when determining management strategy 1
Management Algorithm for Single Subtherapeutic INR
- For patients with previously stable therapeutic INRs who present with a single subtherapeutic INR, continue the current warfarin dose without adjustment 1
- Schedule follow-up INR testing within 1-2 weeks to ensure return to therapeutic range 1
- Bridging with heparin is not routinely recommended for patients with a single subtherapeutic INR 1
- Evidence from retrospective studies shows no significant difference in thromboembolic events between patients with a single low INR and those with therapeutic INRs 1
Management of Persistent Subtherapeutic INR
- For patients with persistently subtherapeutic INRs despite good adherence, consider increasing the weekly warfarin dose by 5-20% 3
- More frequent INR monitoring (e.g., 2-4 times per week) is recommended after dose adjustments until stable therapeutic values are achieved 1, 3
- Consider using validated decision support tools (paper nomograms or computerized dosing programs) to guide dosing adjustments 1
Special Considerations
- For patients with mechanical heart valves, a higher risk of thromboembolism exists with subtherapeutic INRs, but bridging is still not routinely recommended for a single subtherapeutic value 1
- For patients with recent thromboembolism or very high-risk conditions, individual risk assessment may warrant more aggressive management 1
- Patients on high-intensity regimens (target INR 2.5-3.5) receiving ≤6 mg/day of warfarin have >50% risk of subtherapeutic INRs and may require more frequent monitoring 4
Investigating Causes of Subtherapeutic INR
- Evaluate for medication interactions that may decrease warfarin's effect 2
- Assess for dietary changes, particularly increased vitamin K intake or high-protein diets 5
- Consider tobacco use, including smokeless tobacco, which can lead to warfarin resistance 6
- Review patient adherence to prescribed regimen 7
Common Pitfalls to Avoid
- Unnecessarily administering bridging therapy for a single subtherapeutic INR, which increases bleeding risk without clear benefit 1
- Making large dose adjustments based on a single subtherapeutic INR reading, which can lead to INR instability 1, 3
- Failing to identify and address the underlying cause of the subtherapeutic INR 7
- Not considering patient-specific factors that may affect warfarin metabolism and dosing requirements 2
Follow-up Recommendations
- For patients with a single subtherapeutic INR who have returned to therapeutic range, resume regular monitoring schedule 1
- For patients with consistently stable INRs, INR testing frequency can be extended up to 12 weeks 1
- Consider more systematic and coordinated management of oral anticoagulation therapy, incorporating patient education and systematic INR testing 1