The Effects of Prescription Drug Copayments on Statin Adherence

Published Online: September 01, 2006
Teresa B. Gibson, PhD; Tami L. Mark, PhD; Kimberly A. McGuigan, PhD; Kirsten Axelsen, MS; and Shaohung Wang, PhD

Background: High copayments may present a barrier to medication adherence among patients with chronic conditions such as hyperlipidemia.

Objective: To assess the effects of statin copayments on statin adherence among individuals with employer-based insurance.

Study Design: We used a cross-sectional time-series design, with patient as the cross section and month as the time unit.

Methods: Medical and pharmacy claims among continuously enrolled statin users were selected from the 2000-2003 Medstat MarketScan database. Generalized estimating equation models were used to estimate the effects of copayment changes on statin adherence. Adherence was derived from the medication possession ratio, which represents the percentage of days on therapy each month. Separate estimates were obtained for new statin users (n = 142 341) and for continuing statin users (n = 92 344).

Results: Higher copayments were associated with lower statin adherence rates. A 100% index copayment increase had a larger effect on monthly adherence (2.6 and 1.1 percentage point decreases in adherence among new users and continuing users, respectively [both P < .01]) than a 100% copayment increase over time (a 1.1 percentage point decrease among new users [P < .01] and a nonsignificant decrease among continuing users). In all models, new statin users were more price sensitive than continuing users.

Conclusions: High copayments are a financial barrier to statin adherence. The index copayment amount can affect compliance with statin use. Given the relationship between statin use and decreased frequency of cardiovascular events and procedures, the implications of high copayments should be considered by policy makers.

(Am J Manag Care. 2006;12:509-517)

Prescription drug spending continues to rise in the United States, with drug expenditures increasing at a double-digit annual rate for most of the past decade.1 Prescription drug copayments have also increased as employers and other health plan managers attempt to contain prescription drug costs.2 Patient cost sharing (ie, the price paid by the patient [eg, copayments]) is likely to continue to rise. Recent surveys reveal that many firms intend to continue to increase cost sharing in the near future.2,3

Higher prescription drug copayments are associated with lower consumption of prescription drugs and are of concern because they may also lead patients with chronic conditions to decrease utilization of maintenance drugs.4,5 In practice, copayments may encourage nonadherent behaviors such as skipping doses or stopping a medication altogether.6,7

3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin) therapy is a widely accepted treatment for patients with high cholesterol. Clinical trials (eg, the National Cholesterol Education Program Expert Panel,8 Scandinavian Simvastatin Survival Study,9 Air Force/Texas Coronary Atherosclerosis Prevention Study,10 and West of Scotland Coronary Prevention Study11) demonstrate benefits such as decreases in mortality and morbidity associated with statin therapy. Moreover, the extent of risk reduction increases with the amount of time on statin therapy.12 Retrospective studies show that higher statin adherence rates are associated with lower medical care costs,13 fewer hospitalizations,13,14 fewer emergency department visits,14 and higher rates of low-density lipoprotein (LDL) cholesterol goal attainment15 among patients with high cholesterol.

Previous studies14-17 demonstrate that higher prescription drug copayments are associated with lower statin adherence. Schultz and colleagues15 analyzed refill patterns among commercially insured patients from 23 independent practice association health plan affiliates of a single national managed care organization who were new users of statins. The authors found that a $1 statin copayment increase was associated with a 1% decrease in the odds of being adherent in the year following treatment initiation (with adherence defined as a daily dose of medication available for =80% of the days covered by a prescription). Among 216 patients in a Midwestern managed care organization with a history of acute myocardial infarction or other atherosclerotic event, Coombs and colleagues16 observed that statin adherence (defined as the percentage of days covered) would increase from 0.80 to 0.88 if copayments were cut in half. Ellis and colleagues17 found that statin adherence (defined as =10% of days without medication) decreased as copayments increased among commercially insured patients in a Midwestern managed care organization. For example, when copayments increased from less than $10 to at least $10 but less than $20, the percentage of patients who were compliant with statin use decreased by 10% from the first prescription fill until almost 4 years later (the maximum patient follow-up). Finally, in a study of patients enrolled in 88 health plans, Goldman and colleagues14 reported that the percentage of fully compliant patients (ie, those with a medication possession ratio [MPR] =80%) in the year after statin initiation decreased between 6 and 10 percentage points when copayments for cholesterol-lowering drugs were doubled.

The objective of our study was to examine the effects of prescription drug copayments on statin adherence across a variety of health plans. We analyzed the healthcare experience of patients over time using a panel data framework, with patient as the cross section and month as the time unit. This allowed identification of longitudinal and cross-sectional price effects, as well as inclusion of a person-level effect that controlled for unobserved patient characteristics (heterogeneity) such as the propensity to be adherent to a treatment regimen. In addition, a panel data framework enabled us to delineate the price effects by examining the effects of the index copayment amount compared with the effects of the copayment changes over time. Because new users may respond differently to price changes than established users, we also estimate the magnitude of the price effects on new users of statins vs continuing users of statins.


Data Source

The 2000-2003 Medstat MarketScan database was used for this study. More than 45 large firms and more than 100 benefit plan offerings were represented in the database. Patients 18 years and older who were continuously enrolled for 48 months from 2000 through 2003 and who had at least 1 statin prescription fill between 2001 and 2003 were selected for the analysis. Patients with an indication of pregnancy (which can affect adherence) during the study period were excluded from the analysis.

Patient Selection

We analyzed the adherence patterns of new users separately from those of continuing users of statins, because new users are more likely to go through a testing stage to establish treatment efficacy or presence of adverse effects. A total of 142 341 patients who had not filled a statin prescription in the year before the index date (the index date was the date of the first statin fill for new users and was assigned as January 1, 2001, for continuing users) were classified as new (or incident) statin users. There were 92 344 continuing (or prevalent) users.

Study Design

In this study, we followed up a cohort of patients from the index date through December 31, 2003. We used a cross-sectional time-series design, with patient as the cross section and month as the time unit for the adherence analysis. Each continuing user contributed 36 monthly adherence observations. On average, each new user contributed 19 monthly adherence observations.

Explanatory Variables

We developed a model of adherence using the following categories of explanatory variables: copayments, health plan type, comorbidities, time variables, sociodemographic characteristics, and coronary heart disease (CHD) prevalence.17,18 The key explanatory variable was the statin copayment associated with the patient's health plan offering. For each month of the study, copayments were expressed as an amount per day supplied (in 2003 dollars) and were calculated based on the copayment on each pharmaceutical claim record.

Statin copayments were modeled in 2 ways. First, the mean statin copayment in effect during each month was included in the models. Second, to estimate the effects of the index copayment amount vs the effects of copayment amount changes, copayments were modeled as (1) the index copayment (ie, the copayment at the index date) or (2) the dollar amount of copayment change over time relative to the index copayment amount. Because higher physician office visit copayments are associated with lower prescription drug utilization, 19 health plan-level office visit copayments were included in the models.

Sociodemographic variables included sex, age, geographic region (Northeast, North Central, South, or West), urban residence (residence within a metropolitan statistical area), and an indicator of employee vs spouse/dependent status. Patient age was modeled using 2 linear spline variables to assess potential effects of aging and Medicare eligibility. The first variable was age in years up to and including 65 years old, and the second variable was age in years over 65. Information on patient race/ethnicity was unavailable. The median household annual income was assigned based on patient ZIP code according to the 2000 US Census. Patients were grouped into high, medium, or low annual income levels by dividing the distribution of incomes among the study sample into thirds, with a low income representing less than $25 000, a medium income representing $25 000 to $49 930, and a high income representing greater than $49 930. The percentage of college graduates was available from the US Census information but was highly collinear with annual income and was not included in the models. Health plan type was classified as comprehensive, health maintenance organization, capitated point of service, noncapitated point of service, or preferred provider organization.

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