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J Gen Intern Med. 2013 Jul;28(7):876-85. doi: 10.1007/s11606-013-2342-3. Epub 2013 Feb 13.

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

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  • 1Center for Organization, Leadership and Management Research, VA Boston, Boston, MA, USA. nsacks@bu.edu



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.


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.


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


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 %).


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.

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