Abstract
BACKGROUND: Increasing patient cost sharing is a commonly employed mechanism to contain health care expenditures. OBJECTIVE: To explore whether the impact of increases in prescription drug copayments differs between high- and low-income areas. DESIGN: Using a database of 6 million enrollees with employer-sponsored health insurance, econometric models were used to examine the relationship between changes in drug copayments and adherence with medications for the treatment of diabetes mellitus (DM) and congestive heart failure (CHF). SUBJECTS: Individuals 18 years of age and older meeting prespecified diagnostic criteria for DM or CHF were included. MEASUREMENTS: Median household income in the patient's ZIP code of residence from the 2000 Census was used as the measure of income. Adherence was measured by medication possession ratio: the proportion of days on which a patient had a medication available. RESULTS: Patients in low-income areas were more sensitive to copayment changes than patients in high- or middle-income areas. The relationship between income and price sensitivity was particularly strong for CHF patients. Above the lowest income category, price responsiveness to copayment rates was not consistently related to income. CONCLUSIONS: The relationship between medication adherence and income may account for a portion of the observed disparities in health across socioeconomic groups. Rising copayments may worsen disparities and adversely affect health, particularly among patients living in low-income areas.