Cost sharing and decreased branded oral anti-diabetic medication adherence among elderly Part D Medicare beneficiaries

Abstract

Background: Although the Medicare Part D coverage gap phase-out should reduce cost-related nonadherence (CRN) among seniors with diabetes, preferential generic prescribing may have already decreased CRN, while smaller numbers of patients using more costly branded oral anti-diabetic (OAD) medications remain vulnerable to CRN.

Objective: To estimate the effects of cost sharing in the Part D standard (non-LIS) benefit on adherence to different OAD classes, comparing two classes dominated by inexpensive generic medications and two by more costly branded medications.

Design and patients: Retrospective cohort study using dispensed prescription data for elderly non-LIS (N=81,047) and LIS (low-income subsidy) (N=150,359) beneficiaries using same class OAD(s) in 2008 and 2009. Logistic regression modeled non-LIS likelihood; LIS and non-LIS patients matched using propensity outcome (N=38,054). Logistic regression, controlling for demographic and health status characteristics, modeled effects of non-LIS coverage on 2009 OAD class adherence.

Main measures: Main outcome measures were within-class OAD coverage year adherence, with patients considered adherent when days supplied to calendar days ratio at least 0.8.

Key results: Non-LIS patients had 0.52 and 0.57 times the odds of branded-only DPP-4 Inhibitor (N=1,812; 95 % CI: 0.43, 0.63; P<0.001) and Thiazolidinedione (TZD) (N=6,290; 95 % CI: 0.52, 0.63; P<0.001) adherence. Most patients (N=32,510; 82 %) used OADs in primarily generic classes, where we found no significant (Biguanides; N=21,377) or small differences (Sulfonylureas/Glinides [N=19,240; OR: 0.91; 95 % CI: 0.86, 0.97; P=0.002]) in adherence odds. Crude adherence rates were sub-optimal when CRN was not a factor (Non-LIS/LIS: Biguanides: 65 %/65 %; Sulfonylureas/Glinides: 66 %/68 %; LIS: DPP-4 Inhibitors: 66 %; TZDs: 67 %).

Conclusions: Gap elimination would not affect generic, but should reduce branded OAD CRN. Branded copayments may continue to lead to CRN. Policy initiatives and benefit changes targeting both cost deterrents for patients with more complex disease and non-cost generic OAD underuse are recommended.

Figure 1.

Cohort of diabetes patients.

Figure 2.

Proportion of generic and branded prescriptions dispensed by oral anti-diabetic class.

Figure 3.

Use and crude adherence rates by oral anti-diabetic class.