Background: Accurate estimates of the effects of cost sharing on adherence to medications prescribed for use together, also called concurrent adherence, are important for researchers, payers, and policymakers who want to reduce barriers to adherence for chronic condition patients prescribed multiple medications concurrently. But measure definition consensus is lacking, and the effects of different definitions on estimates of cost-related nonadherence are unevaluated.

Objectives: To (a) compare estimates of cost-related nonadherence using different measure definitions and (b) provide guidance for analyses of the effects of cost sharing on concurrent adherence.

Methods: This is a retrospective cohort study of Medicare Part D beneficiaries aged 65 years and older who used multiple oral antidiabetics concurrently in 2008 and 2009. We compared patients with standard coverage, which contains cost-sharing requirements in deductible (100%), initial (25%), and coverage gap (100%) phases, to patients with a low-income subsidy (LIS) and minimal cost-sharing requirements. Data source was the IMS Health Longitudinal Prescription Database. Patients with standard coverage were propensity matched to controls with LIS coverage. Propensity score was developed using logistic regression to model likelihood of Part D standard enrollment, controlling for sociodemographic and health status characteristics. For analysis, 3 definitions were used for unadjusted and adjusted estimates of adherence: (1) patients adherent to All medications; (2) patients adherent on Average; and (3) patients adherent to Any medication. Analyses were conducted using the full study sample and then repeated in analytic subgroups where patients used (a) 1 or more costly branded oral antidiabetics or (b) inexpensive generics only.

Results: We identified 12,771 propensity matched patients with Medicare Part D standard (N = 6,298) or LIS (N = 6,473) coverage who used oral antidiabetics in 2 or more of the same classes in 2008 and 2009. In this sample, estimates of the effects of cost sharing on concurrent adherence varied by measure definition, coverage type, and proportion of patients using more costly branded drugs. Adherence rates ranged from 37% (All: standard patients using 1+ branded) to 97% (Any: LIS using generics only). In adjusted estimates, standard patients using branded drugs had 0.63 (95% CI = 0.57-0.70) and 0.70 (95% CI = 0.63-0.77) times the odds of concurrent adherence using All and Average definitions, respectively. The Any subgroup was not significant (OR = 0.89, 95% CI = 0.87-1.17). Estimates also varied in the full-study sample (All: OR = 0.79, 95% CI = 0.74-0.85; Average: OR = 0.83, 95% CI = 0.77-0.89) and generics-only subgroup, although cost-sharing effects were smaller. The Any subgroup generated no significant estimates.

Conclusions: Different concurrent adherence measure definitions lead to markedly different findings of the effects of cost sharing on concurrent adherence, with All and Average subgroups sensitive to these effects. However, when more study patients use inexpensive generics, estimates of these effects on adherence to branded medications with higher cost-sharing requirements may be diluted. When selecting a measure definition, researchers, payers, and policy analysts should consider the range of medication prices patients face, use a measure sensitive to the effects of cost sharing on adherence, and perform subgroup analyses for patients prescribed more medications for which they must pay more, since these patients are most vulnerable to cost-related nonadherence.