Purpose: To examine the impact of two methods when estimating refill adherence in patients using bisphosphonates with different dosing regimens.
Methods: In the Swedish Prescribed Drug Register, 18,203 new users of bisphosphonates aged 18-85 years were identified between 1 July 2006 and 30 June 2007 and followed for a maximum of 2 years. The patients were categorised based on dosing regimen: one tablet daily, one tablet weekly, switching between these regimens, and other regimens. Refill adherence was estimated with Continuous measure of Medication Acquisition (CMA, adherent if CMA ≥ 80 %) and the maximum gap method (adherent if gaps <45 days). Differences in adherence between patients in the groups were assessed with logistic regression models controlling for confounding factors.
Results: The proportion of patients classified as adherent was higher using CMA compared with patients classified as adherent using the maximum gap method. Patients on one tablet weekly had significantly lower adherence compared with patients on one tablet daily in the main analyses of both methods (the maximum gap method: 73 % vs. 80 %; adjusted OR=0.71; 95 % CI 0.57-0.89 and CMA: 84 % vs. 88 %, adjusted OR=0.75; 95 % CI 0.57-0.99). Patients using the other two dosing regimens had significantly lower adherence compared with patients on one tablet daily using both methods.
Conclusion: Choice of method has an impact on the estimates of refill adherence to bisphosphonates. Patients on one tablet weekly dosing had lower adherence compared with patients on one tablet daily dosing using both methods.