Background: Treatment with warfarin greatly reduces the risk of stroke related to atrial fibrillation, but will not be effective unless patients adhere to treatment. Lack of fixed dosing makes it difficult to objectively estimate adherence to treatment from prescription data.
Objective: To evaluate two methods that assess adherence to warfarin from prescription data.
Design: Retrospective study of Swedish health care registers.
Patients and methods: Age- and sex-specific dose requirements were determined from approx. 1 million blood tests and dosing instructions. By applying these dosages to 163,785 warfarin-treated patients with atrial fibrillation, we calculated the quantity of warfarin that was needed to keep these patients on effective treatment during a mean follow-up of 3.9 years and compared that with the dispensed quantities. The ratio of available drug/time at risk constitutes a measure of adherence on group level. In addition, time intervals between refills were used to assess discontinuation.
Results: Both methods showed that 45% of the patients did not have enough warfarin to last 80% of the time at risk. Between 16 and 21% of the patients discontinued within the first year, followed by 8-9% annually during the following years. Patients with high bleeding risk and patients with low embolic risk showed lower endurance.
Conclusions: Adherence to treatment with warfarin can be estimated on group level from prescription data and may be useful for comparison of adherence with warfarin and new oral anticoagulants. When applied to a large warfarin-treated cohort with atrial fibrillation, we found that adherence is low and that measures aiming for improvements are needed .