From the FDA Drug Label
The dosage and administration of warfarin sodium tablets must be individualized for each patient according to the particular patient’s PT/INR response to the drug. The dosage should be adjusted based upon the patient’s PT/INR. The dose of warfarin should be adjusted to maintain a target INR of 2.5 (INR range, 2.0 to 3.0) for all treatment durations.
For a patient on Warfarin with an INR of 1.1, which is below the target range of 2.0 to 3.0, the dosage of warfarin should be increased to achieve a therapeutic level of anticoagulation. The exact increase in dosage cannot be determined without more information about the patient's current dosage and response to warfarin.
- Key considerations:
- The patient's PT/INR response to the drug
- The target INR range of 2.0 to 3.0
- The need for individualized dosage adjustment
- Recommendation: Increase the warfarin dosage and monitor the patient's INR closely to ensure it reaches the target range, based on the patient’s PT/INR response to the drug 1.
From the Research
Management of Subtherapeutic Anticoagulation
- The management of subtherapeutic anticoagulation in patients on Warfarin with an INR of 1.1 is crucial to prevent thromboembolic complications 2.
- Research suggests that patients with stable INRs while receiving warfarin who experience a significant subtherapeutic INR value have a low risk of thromboembolism in the ensuing 90 days 2.
- The risk of thromboembolism is similar to that observed in a matched control population in whom therapeutic anticoagulation was maintained 2.
Factors Influencing Warfarin Dose Response
- Patient factors such as age, body weight, nutritional status, acute and chronic disease states, and changes in concomitant drug therapy and diet can influence warfarin dose response 3.
- Pharmacogenetics also plays a role in optimizing warfarin therapy 3.
- The narrow therapeutic index of warfarin makes optimization of warfarin therapy challenging 3.
Clinical Implications of Out-of-Range INR
- A sub-therapeutic INR may not protect against thromboembolic complications, while a supra-therapeutic INR puts patients at risk of bleeding 4.
- Careful monitoring of the INR is essential, especially in geriatric or cancer populations who are at an increased risk of major hemorrhage 4.
- Optimization of the risk-benefit ratio is crucial to maximize efficacy and safety of warfarin therapy 4.
Guidelines for Warfarin Therapy
- The anticoagulant effect of warfarin should be kept at an INR of about 2.5 (desirable range, 2.0-3.0) 5.
- The risk of bleeding increases exponentially with INR and becomes clinically unacceptable once the INR exceeds 5.0 5.
- Warfarin therapy should be continued for around six weeks for symptomatic calf vein thrombosis, and for 3-6 months after proximal deep vein thrombosis (DVT) that occurs after surgery or limited medical illness 5.