Association of Copayment with Likelihood and Level of Adherence in New Users of Statins: A Retrospective Cohort Study

BACKGROUND: Statins remain a fundamental component of pharmacologic therapy for hyperlipidemia. Health benefits of statin therapy are jeopardized when adherence is reduced. OBJECTIVES: To (a) assess the association between copayment and copayment type on statin adherence using 2 different thresholds of adherence and (b) identify the incremental change in statin adherence associated with presence of copayment and copayment type. METHODS: We executed a retrospective cohort study of new users of statins with dyslipidemia from the Veterans Health Administration (VHA) within the Veterans Integrated Service Network 22 who initiated a statin between November 30, 2006, and December 2, 2007. We used exposure categories of Any Copayment versus No Copayment, indicating a patient had a copayment or had no copayment in order to obtain medications, respectively. As a separate analysis, we varied the exposures to the standard VHA copayment categories: (a) Service-Connected (SC) Copayment (patients with service-related injury), (b) Non-Service-Connected (NSC) Copayment (patients without a service-related injury), and (c) No Copayment. Using each set of exposures, we conducted separate multiple logistic regression analyses using 2 different adherence outcomes based on medication possession ratio (MPR) threshold: (1) adherence defined as MPR ≥ 0.8 and (2) adherence defined as MPR ≥ 0.9. We then proceeded with multiple linear regression models to determine the incremental change in MPR associated with the 2 sets of exposures. Subjects were required to be enrolled in VHA services for at least 2 years prior to index date and throughout the 1-year study period. RESULTS: A total of 4,886 subjects were identified for analysis based on the inclusion and exclusion criteria. Patients who did not pay a copayment for their statin medications were more likely to have adherence rates of ≥ 0.8 MPR and ≥ 0.9 MPR relative to the No Copayment Group with odds ratios (OR) of 1.19 (95% CI = 1.03-1.37) and 1.28 (95% CI = 1.11-1.48), respectively. The second analysis applied the VHA exposure categories of SC Copayment, NSC Copayment, and No Copayment. Using the 0.8 MPR or greater adherence threshold, the No Copayment group was associated with an increased likelihood of adherence versus the SC Copayment category as reference group with an OR of 1.31 (95% CI = 1.10-1.58). The NSC Copayment was associated with a nonsignificant increase in odds of adherence at the 0.8 MPR level or greater with OR of 1.12 (95% CI = 0.98-1.39). Using the 0.9 MPR level or greater, adherence threshold findings were similar. The No Copayment group produced an OR of 1.42 (95% CI = 1.17-1.71) compared with the SC Copayment group. The NSC Copayment group was associated with a nonsignificant increase in odds of adherence at the 0.9 MPR level or greater with an OR of 1.12 (95% CI = 0.97-1.38).The No Copayment group was associated with an increase in MPR of 0.02 (95% CI = 0.002-0.035) versus the Any Copayment category. Using the VHA copayment categories, we observed an increase in MPR for the No Copayment group versus the SC Copayment group of 0.03 (95% CI = 0.01-0.05). The NSC Copayment group was associated with a nonsignificant increase in MPR versus the SC Copayment group of 0.02 (95% CI = -0.003-0.036). CONCLUSIONS: Patients without out-of-pocket payments for their statins were more likely to adhere to therapy. Patients who pay a copayment for their statin medications were also compared with each other based on whether they (a) received any of their nonstatin prescriptions without a copayment or (b) paid a copayment on all of their prescriptions including statins. Our findings suggest that, among those that pay for their statins, patients are less adherent to their statins if other medications they are prescribed are copayment free. Thus, patient consumption behavior may be influenced by the relative cost of medications in patient prescription lists. Additional counseling on the necessity of adherence should be given to patients paying a copayment for their statin prescriptions.