The Effects of Prescription Drug Copayments on Statin Adherence | Page 2

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

As a surrogate measure for CHD prevalence (and secondary prevention), we indicated the presence of the following cardiovascular diagnoses in a rolling 1-year lag before each month of the study: acute myocardial infarction, angina, coronary artery bypass graft, chronic ischemic heart disease, coronary atherosclerosis, other ischemic heart disease, and percutaneous transluminal angioplasty.17,18 Comorbidities were measured using the Charlson Comorbidity Index (based on medical claims data and adapted from D'Hoore et al20) and using separate indicators for the common comorbid conditions of anxiety, dementia, depression, diabetes mellitus, and hypertension. CHD prevalence and comorbidities were also measured in a rolling 1-year lag. Physician-related variables included the number of different physicians seen by each patient and an indicator for a specialist visit to a cardiologist, endocrinologist, or nephrologist. The specialist visit indicator serves as a proxy for increased disease severity and complexity of comorbidities such as kidney disease. Medication-related variables included the number of different medications that were prescribed across all disease states, perhaps indicating the burden of managing prescriptions. An indicator for any prior mail-order use, which may represent a propensity to invest in health, was also included. An indicator was also included for the use of ezetimibe, a cholesterol-lowering drug approved by the Food and Drug Administration in October 2002, which can be used alone or in conjunction with statins. Physician and medication variables were also measured in a rolling 1-year lag.

Studies15,17 indicate that the use of LDL cholesterol testing is associated with higher adherence levels. Given the rolling panel design of our study and the use of LDL cholesterol testing for monitoring during statin therapy, the use of LDL cholesterol testing was likely to be endogenous with adherence. Therefore, we chose to use other predictors of adherence.

Because statin adherence decreases over time,18,21 the amount of time on statin therapy was measured as the number of months since the index date (0-3, 4-6, 7-9, 10-12, 13-18 months, or =19 months). For new users, the year of the index prescription was included to account for treatment pattern changes over time.

Outcome Measures

Adherence was derived from the MPR, calculated as the usable days supplied from among all statin refills in the month, divided by the number of days in the month. To calculate the monthly MPR, each day was evaluated as covered or as not covered by a prescription fill. Covered days comprised the time between the fill date and the end date of the prescription (ie, the fill date plus the days' supply of the prescription). If a refill for the same statin occurred before the end date of the previous prescription, the days' supply for the new prescription was appended to the end date of the previous prescription. If a refill for a statin occurred after the end date of the previous prescription, the days between the 2 prescriptions were considered uncovered days. If a patient received a prescription for a new statin drug while he or she had a usable supply of another statin on hand, no overlap occurred, and counting commenced from the fill date of the new statin. Few patients (approximately 10%) switched statin drugs during the study.

The MPR measures statin adherence and does not include switches to other cholesterol-lowering medications; therefore, the statin adherence rates reported herein are somewhat lower than the adherence rates associated with all cholesterol-lowering medications. We were unable to measure adherence behavior while patients were hospitalized, although hospitalization occurred infrequently (approximately 2% of patients each month).

The monthly MPR distribution was largely bimodal, so we expressed adherence as an MPR of 80% or higher.15,21 Similar thresholds have been established as beneficial in clinical trials11,22 and in the statin adherence literature.14,15 By setting the adherence level at an MPR threshold of 80%, we allowed for some variation in use. However, we were unable to determine whether patients received drug samples or were splitting pills at the direction of a provider. In a well-insured population such as this, the use of samples is likely to be low. We did not see a large number of patients who had an MPR of 50% (which would indicate splitting pills in half), so the amount of pill splitting was also likely to be low.

Adherence was calculated among patients in each month following the index date through December 31, 2003. This allowed analysis of the actual adherence behavior of patients, including patients with inconsistent adherence patterns.

Modeling Approach

Adherence was modeled as a function of the following covariates: adherenceit = f (sociodemographic characteristicsit, health plan typeit, physician variablesip, medication variablesip, CHD prevalenceip, comorbiditiesip, copaymentit, and time variablesit), where i represents patient; t, month; and p, rolling 1-year lag.

Generalized Estimating Equation models were used to model adherence using a binomial outcome and a logit link.23,24 Standard errors were adjusted for clustering by patient over time using robust standard errors to decrease the effects of unknown heteroscedasticity or specification errors.25 Separate models were estimated for new users and for continuing users.

RESULTS

A total of 234 685 statin users were identified who met all inclusion criteria. Descriptive characteristics of the new users and the continuing users are given in Table 1. The patient cohort contained approximately 50% more new users (n = 142 341) than continuing users (92 344), with approximately one third of new users initiating statin therapy in each study year.



Among new users, 51.3% were men, and 48.7% were women (mean age, 57.0 years) (Table 1). New users were more likely to be employees (vs spouses/dependents), to reside in the South (vs other geographic regions), and to be enrolled in a preferred provider organization (vs other health plan types). Almost one fifth (18.1%) had a diagnosis of angina, and 12.5% had a diagnosis of coronary atherosclerosis in the year before the index prescription. Other CHD diagnoses (acute myocardial infarction, coronary artery bypass graft, chronic ischemic heart disease, and percutaneous transluminal angioplasty) were each present in fewer than 5% of patients. Comorbidities were prevalent among new users. Almost 44% of patients had a diagnosis of hypertension, and 19.6% had a diagnosis of diabetes mellitus in the year before the index prescription. New users filled a mean of 6.4 medications in the year before the index prescription, and 41.6% had used a mail-order pharmacy to fill a prescription in the previous year. The mean statin copayment was approximately $15 per prescription for a 30-day supply ($0.51 per day) (the models and the tables use a daily copayment rate; all copayments are reported elsewhere in the article as the amount per 30-day supply) at the time of the index prescription. The mean statin copayment increase was $1.50, including health plans that did and did not increase copayments, which aids identification of the effects of copayments on adherence. The maximum copayment increase was approximately $25.

Among continuing users, 55.4% were men, and 44.6% were women (mean age, 64.1 years) (Table 1). Continuing users were more likely to be employees (vs spouses/dependents), to reside in the South (vs other geographic regions), and to be enrolled in a capitated point-of-service health plan (vs other health plan types). Approximately one fifth (20.1%) had a diagnosis of coronary atherosclerosis, and 15.5% had a diagnosis of angina in the year before the index prescription. Almost 44% of patients had a diagnosis of hypertension, and 19.5% had a diagnosis of diabetes mellitus in the year before the index date. Continuing users filled a mean of 8.8 medications in the year before the index date, and 58.1% had used a mail-order pharmacy to fill a prescription in the year before the index date. The mean statin copayment was approximately $12 per prescription for a 30-day supply ($0.41 per day) at the index date. The mean statin copayment increase was $1.80, including health plans that did and did not increase copayments. The maximum copayment increase was approximately $39.

The mean monthly adherence, measured across all postindex months, was higher among continuing users, who had 71% of months with an MPR exceeding 80% (Table 1). New users had 48% of months with an MPR exceeding 80%.

Adherence Models

Results from the Generalized Estimating Equation models of adherence revealed that among new users higher copayments were associated with lower monthly adherence (Table 2). A $10 index copayment increase was associated with a 3% decrease (P < .01) in the odds of adherence. In percentage terms, a 100% index copayment increase was associated with a 1.2 percentage point decrease in the probability of monthly adherence.

 

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