Improved Stability in INR with Coumadin for a Patient Requiring Very Low Warfarin Doses

Douglas C Anderson Jr, Phillip Poulos, and Bernard P Scoggins

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Objective: To describe a patient who required very low doses of warfarin but who could not achieve therapeutically stable anticoagulation until treatment was switched from generic to branded warfarin.

Case Summary: A 53-year-old male with a history of hypertension, coronary artery disease, heart failure, and mild renal insufficiency was diagnosed with atrial fibrillation during a routine examination. Anticoagulation was started with warfarin 5 mg daily. The patient presented to the anticoagulation clinic 2 days later with an international normalized ratio (INR) of 5.4 and no signs or symptoms of bleeding. Warfarin was held for 3 days until the INR was <3. Warfarin was then restarted at 2.5 mg daily. Over the next week his INR rose to 4.8; warfarin was again held and restarted at 1.5 mg/day. Although the warfarin dose was eventually titrated to 6.5 mg/wk, the INR never stabilized and the patient only had consecutive therapeutic INRs twice in 1 year and only one 3-month period with therapeutic INRs not requiring any dosage adjustments. The decision was made to switch from the generic product to Coumadin to try to improve stability. Several dosage adjustments were made over the 6 weeks following the switch before the INR was stabilized with 7 mg/wk. Subsequent INRs ranged from 2.0 to 2.2 in the 3 months after the switch, during which time INRs were measured at least weekly. Over 14 months after the switch, the patient did not have an INR outside of the therapeutic range.

Discussion: Many patients are able to take generic warfarin without problems. However, case reports have documented occasional patients who have had problems after switching to a generic product. Large cohorts have been switched to generic warfarin without experiencing significant changes in therapeutic control. However, 1 cohort study found that patients who required very low warfarin dosages had significant changes in their INR after switching to a generic product.

Conclusions: Patients who require very low warfarin dosages to consistently maintain therapeutic anticoagulation may experience less therapeutic stability while on a generic product.

J Pharm Technol 2010;26:220-3