The benefits of drug therapy to diabetic patients in terms of glycemic control, microvascular complications, cardiovascular event risk, mortality, and quality of life have been well established by clinical trial data. However, it has been a challenge to quantify the relationship between adherence and outcomes such as glycemic control, disease-related events, hospitalizations, cost, and quality of life. This article provides a comprehensive summary of empirical studies that examine the associations between adherence and glycemic control, health care utilization, quality of life, and mortality in patients with diabetes. It is intended to provide a framework for researchers interested in conducting studies to improve their understanding of the value of medication adherence for patients with diabetes. Relevant published articles were identified through searches of the National Center for Biotechnology PubMed database. Medical subject heading (MESH) terms diabetes mellitus, hypoglycemic agents, and insulin, were each combined with the MESH term medication adherence and with the subheadings economics, prevention and control, psychology, statistics and numerical data, therapy, adverse effects, therapeutic use, and administration and dosage, where available. Studies were included if they met the following criteria: (1) analyzed empirical data on some measure of patient adherence to diabetes pharmacotherapy; (2) described methods for measuring patient adherence; (3) evaluated economic, clinical, or humanistic outcomes related to diabetes; and (4) had as a goal of the research to evaluate the link between patient adherence and outcomes (as a primary or secondary objective). The data from the articles meeting these criteria were then abstracted, including mention of the specific interventions being compared, specific methods for measuring adherence, outcomes compared between adherent and nonadherent patients and how these outcomes were measured, and information on variables that were adjusted for in predictive and causal multivariable models. A total of 37 articles that met all 4 criteria in this review underwent data extraction. Of these studies, 22 (59%) used objective measures to assess adherence, with 1 study using pill counts to assess adherence and 21 using either pharmacy claims or similar refill records to assess refill behavior. The remaining 15 (41%) studies used a wide variety of subjective patient-reported adherence assessments. The majority (13/23 [57%]) of the glycemic control studies reported that improved adherence was associated with better glycemic control. The ability to draw a distinction between adherence and glycemic control tended to occur more frequently [7/9 (78%)] among studies that characterized adherence in terms of prescription refills compared with studies that used various constructs for patient-reported adherence measures. Based on the literature, better adherence was found to be associated with improved glycemic control and decreased health care resource utilization. There was no consistent association between improved adherence and decreased health care costs. Little data were available on the association between adherence and quality of life. Copyright © 2011 Elsevier HS Journals, Inc. Published by EM Inc USA. All rights reserved.
