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Multiple Medication Adherence and its Effect on Clinical Outcomes Among Patients With Comorbid Type 2 Diabetes and Hypertension

An, JaeJin PhD*,†; Nichol, Michael B. PhD

doi: 10.1097/MLR.0b013e31829fa8ed
Original Articles

Objective: To investigate multiple medication adherence (MMA) and its impact on microvascular and macrovascular complications using instrumental variables (IVs).

Research Design: A retrospective observational study was conducted using administrative claims and electronic medical records from a large physician group in Southern California (N=2334).

Subjects: We identified individuals between January 2006 and June 2009 newly starting oral diabetes (DM) or hypertension (HTN) medications with preexisting comorbid HTN or DM prescription history.

Measures: MMA was defined as a proportion of days covered where both DM and HTN medications were simultaneously available over a 33-month follow-up period. Microvascular or macrovascular complications included myocardial infarction, stroke, renal failure, and diabetic retinopathy. Multivariable logistic regressions and an IV estimation using physician-related variables were implemented.

Results: MMA was supoptimal as the mean (SD) proportion of days covered was 0.53 (0.32). Patients were more adherent to medications for a preexisting condition in comparison with those for the newer disease. Older age, number of index medications [OR (95% CI)=1.36 (1.22–1.52)], receiving care from a physician who prescribed statin more frequently [OR (95% CI)=2.63 (1.67–4.14)], and receiving care from the same physician for both DM and HTN [OR (95% CI)=1.57 (1.08–2.27)] were significant factors of being adherent. Using physician-related IVs, MMA reduced microvascular and macrovascular complications. The increase in MMA from 50% to 80% reduced the average predicted probability of microvascular or macrovascular complication rate by 29.5%.

Conclusions: Adherence to medications for DM and HTN were differed and higher MMA reduced microvascular or macrovascular complications when controlling for endogeneity bias.

Supplemental Digital Content is available in the text.

*Pharmacy Analytical Services Research Group, Kaiser Permanente Southern California, Downey

Schaeffer Center for Health Policy and Economics, University of Southern, California, Los Angeles, CA

Supplemental Digital Content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Website,

J.A.: developed research ideas, conducted statistical analyses, and drafted the manuscript and M.B.N.: contributed to the study design, interpretation of the study results, and critical revision of the manuscript.

Presented in part at the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) 16th Annual International Meeting, Baltimore, MD, May 21 to 25, 2011.

The authors declare no conflict of interest.

Reprints: JaeJin An, PhD, Pharmacy Analytical Services Research Group, Kaiser Permanente Southern California, 12254 Bellflower Blvd. Downey, CA 90242. E-mail:

© 2013 by Lippincott Williams & Wilkins.