Published Online: December 01, 2008
Rebecca L. Sedjo, PhD; and Emily R. Cox, PhD
Objective: To assess the impact of a decrease in statin copayments on medication adherence and demand for statins.
Study Design: Quasi-experimental, pre/post design.
Methods: Patients in more than 700 health plans from June 2005 to May 2007 were evaluated. The intervention group (n = 13,319) and matched control group (n = 26,569) included patients who had at least 1 branded simvastatin or non-simvastatin statin purchase, respectively, before the simvastatin patent expired in June 2006. Intervention and control patients had to have purchased at least 1 generic simvastatin and non-simvastatin statin, respectively, after patent expiration. Intervention patients were matched to control patients up to 1:2 on incident statin use (yes/ no) and pre–patent expiration copay (± $2). Adherence was calculated with the medication possession ratio (MPR). Adjusted and unadjusted changes in MPR were compared between groups. Elasticity of demand for statins was estimated.
Results: A small but statistically significant difference was observed between groups in the change in MPR (intervention = 0.52% adjusted mean increase, control = 2.02% adjusted mean decrease; adjusted P <.01). A marginally higher percentage of intervention patients (10.5%) compared with control patients (10.0%) increased their MPR from =80% in the preperiod to >80% in the postperiod (adjusted P <.01). Elasticity of demand for statins was estimated at 0.02 and -0.02 for the copayment reduction categories of $0 to $5 and >$15, respectively.
Conclusions: Decreasing statin copayments was associated with adherence increases. However, the overall increase in medication adherence was modest and its clinical significance uncertain.
(Am J Manag Care. 2008;14(12):813-818)
Decreasing statin copayments because of the patent expiration of simvastatin has led to increases in therapy adherence, especially among those patients with a decline of $10 or more.
- The overall extent of this increase is moderate and its clinical significance uncertain.
- Given the small identified relationship between decreases in copayments and medication adherence, plan sponsors who wish to lower copayment for their patients should consider the use of cost-effective therapies such as lower cost brands and generics.
In an effort to keep pace with rising prescription drug costs, health plans have implemented various cost-sharing strategies including prescription medication copayments, tiering, and coinsurance. Goldman and colleagues reported in their literature review of 65 studies that a 10% increase in patient cost sharing (eg, copayment increase) would result in a 2% to 6% decline in prescription medication use or expenditures.1 The resulting decline in medication utilization is cause for concern, given the reported relationship between patient adherence and health outcomes.2-5 Understandably, 1 method to improve prescription medication adherence that has received considerable attention among health plan decision makers is to lower patient copayments. Unfortunately, the benefit of this approach for medication adherence has not been evaluated fully.
The 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors (statins) have been shown to be efficacious in reducing morbidity and mortality associated with coronary heart disease.6,7 Several studies have reported a relationship between increasing statin copayments and decreasing adherence.2,5-7 Evidence from these studies has been used to speculate that a similar marginal increase in adherence will result from lowering copayments.8 However, only 1 study to date has provided data to support this hypothesis by examining the relationship between decreasing copayments and statin adherence.9 In this quasi-experimental study of patients enrolled in a disease management program, copayment amounts were decreased from $5 to $0, $25 to $12.50, and $45 to $22.50 for generics, brands, and nonpreferred drugs, respectively, resulting in a 3.4% increase in statin medication adherence.
On June 23, 2006, the patent for the statin Zocor (simvastatin) expired. This patent expiration and ensuing introduction of generic simvastatin resulted in lower copayments for patients who had at least a 2-tiered (brand/ generic) prescription medication benefit and whose refills were automatically converted to the generic. This occurrence provided a naturalistic setting to contrast adherence changes among patients without a copayment decrease (ie, those who were receiving a non-simvastatin brand statin before the patent expiration) and patients with a copayment decrease (ie, those who were receiving the branded simvastatin before the patent expiration). Thus, the purpose of this study was to examine the effect on medication adherence of a decrease in patient copayments and demand for statins after the simvastatin patent expiration.
Prescription claims data from a nationwide population of more than 700 plan sponsors (managed care organizations, insurance carriers, employer groups, third-party administrators, and public sector–sponsored and union-sponsored pharmacy benefit plans) that were enrolled with the pharmacy benefit manager Express Scripts Inc from June 2005 through May 2007 were eligible for this study. Only those plan sponsors that offered integrated prescription coverage that included both a home-delivery and retail benefit within an employerbased market (ie, no Medicare or Medicaid) and did not change their preferred brand copayment during the preindex period and postindex period by more than $3 were eligible for inclusion. In addition, eligibility was limited to patients >18 years of age who were statin purchasers and were continuously eligible during the study period. This study was not submitted to an institutional review board; however, all regulations related to the Health Insurance Portability and Accountability Act were followed.
A quasi-experimental, pre/post, controlled design was used. From eligible plans, 13,319 patients who purchased at least 1 brand simvastatin prescription between June 1, 2005, and August 31, 2005 (preindex period) and at least 1 generic simvastatin prescription between June 1, 2006 and August 31, 2006 (postindex period) were identified as intervention group patients (Figure). A control group of 26,569 patients who purchased at least 1 non-simvastatin brand statin prescription in the preindex period and at least 1 non-simvastatin statin prescription in the postindex period were identified as control group patients. Adherence, as measured by the medication possession ratio (MPR),10 was assessed from the date of the first purchase in the preindex period through the following 270 days (preperiod) and date of the first purchase in the postindex period through the following 270 days (postperiod) (Figure). This schema restricts a patient’s MPR calculation to the dates most proximal to the simvastatin patent expiration.
The control group was matched to the intervention group at a ratio of 2:1 based on incident statin purchaser (yes/no) and preperiod copayment amount (± $2) using the method of Bergstralh and Kosanke.11 To avoid overmatching, only these variables were selected because they have been associated previously with statin adherence.2 Incident statin purchaser was defined as a patient with no statin purchase in the 130 days before his or her first preindex period purchase. Two controls and 1 control were matched to 13,250 and 69 intervention patients, respectively.
The primary outcome measure for this study was the change in adherence. The change in MPR was calculated by subtracting the preperiod MPR from the postperiod MPR. Secondary outcomes include assessment of the percentages of patients who increased their MPR to ≥80% in the postperiod from <80% in the preperiod and decreased their MPR to <80% in the postperiod from ≥80% in the preperiod. Elasticity of demand for statins was estimated between the highest and lowest copayment reduction categories. Comparisons of changes in MPR between various subgroups were made.
Medication possession ratios were calculated for each patient as the sum of the days’ supply for statin purchases in each observation period divided by 270 and then multiplied by 100 to obtain a percentage.10 The days’ supply of the last statin purchase in the observation period was truncated if it went beyond the 270-day follow-up period; thus, MPR values could not exceed 100%. Although the initial statin purchase was brand simvastatin for the intervention group and any other brand statin for the control group, adherence was based on any filled statin prescription in the pre- or postperiod to allow for switching within the statin class.
The MPR in both the pre- and postperiods was categorized as <80% and ≥80%. A chronic disease score (CDS) was calculated using prescription medication purchases from the first 6 months of 2006.12 The CDS provides a numerical accounting of each patient’s baseline health status. The CDS was log transformed because of its nonnormal distribution. Mean copayments during the pre- and postperiods were determined for intervention patients. A change in copayment was determined for the intervention patients by subtracting the postperiod copayment from the preperiod copayment. These values were categorized as ≥$0, declines of $0.01 to $5.00, declines of $5.01 to $10.00, declines of $10.01 to $15.00, and declines of >$15.00. This change was not assessed in the control patients as the percentage of patients with a copayment decrease was negligible.
Baseline differences between the intervention and control groups with respect to age, sex, incident statin purchaser, CDS, preperiod adherence, and preperiod copayment were assessed with adjustment of the standard errors for the intracorrelations of matched patients using multiple linear regression models for continuous variables and logistic regression for categorical variables. The unadjusted relationship of the group on the change in MPR was assessed using multivariate linear regression with adjustment for the intracorrelations of matched patients. The adjusted relationship of the group on the change in MPR was assessed with multivariate linear regression with adjustment for the intracorrelations of matched patients and age, sex, incident statin purchaser, CDS, preperiod MPR, and pre-period copayment. Using this fitted regression model, adjusted mean MPR changes were calculated for the intervention and control groups by setting all covariates to the population mean values.
To further explore the relationship between adherence and copayment amount, another multivariate linear regression model was fit using data from the intervention group only. The 5 levels of change in copayment amount were regressed on change in MPR while adjusting for the covariates listed previously. Adjusted mean MPR changes were calculated for the categories of copayment change by setting all covariates to their mean values. Elasticity of demand was calculated as the percent change in MPR divided by the percent change in copayment. Sensitivity analyses were performed by removing subpopulations of patients including (1) all patients with coinsurance (ie, they may not have experienced as dramatic a decrease in copayment because of market exclusivity of the generic for the first 6 months after patent expiration, when typically the price of the generic is only marginally lower than the multisource brand) and (2) intervention patients who switched from generic simvastatin to a non-simvastatin statin in the postperiod (ie, they may have experienced a copayment increase). All statistical analyses were performed with STATA/SE, version 8.0 (StataCorp LP, College Station, TX) using 2-sided statistical tests with an alpha level of .05.
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