How Does Drug Treatment for Diabetes Compare between Medicare Advantage Prescription Drug Plans (MAPDs) and Stand-Alone Prescription Drug Plans (PDPs)?


Address correspondence to Mujde Z. Erten, Ph.D., Pharmaceutical Health Services Research, School of Pharmacy, University of Maryland Baltimore, Baltimore, MD 21201; e-mail:



To compare the use of guideline-recommended prescription medications for diabetes among Medicare beneficiaries enrolled in stand-alone prescription drug plans (PDPs) with Medicare Advantage prescription drug plans (MAPDs) in the presence of potential selection bias.

Data Sources/Study Setting

Centers for Medicare and Medicaid Services' Chronic Condition Data Warehouse (2006, 2007).

Study Design

Retrospective cross-sectional comparison of drug use and proportion of days covered (PDC) for oral-antidiabetics, ACE-inhibitors/ARBs, and antihyperlipidemics among PDP and MAPD enrollees with diabetes. We estimated “naïve” regression models assuming exogenous plan choice and two-stage residual inclusion (2SRI) models to study endogeneity in choice of Part D plan type.

Data Collection/Extraction Methods

We identified 111,290 diabetics based on ICD-9 codes in Medicare claims from a random 5 percent sample of Medicare beneficiaries in 2005 excluding dual eligibles.

Principal Findings

The naïve regression models indicated lower probability of drug use for oral-antidiabetics (−4 percent; p < .001) and ACE-inhibitors/ARBS (−2 percent; p = .004) among PDP enrollees, but their PDC was higher (3–5 percent) for all drug classes (p < .001). 2SRI models produced no significant differences in any-use equations, but significantly higher PDC values for PDP enrollees for oral-antidiabetics and ACE-inhibitors/ARBs.


We found similar overall use of recommended drugs in diabetes treatment and no consistent evidence of favorable or adverse selection into PDPs and MAPDs.