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Diabet. Med. 27, 210–216 (2010) Aims To assess pharmacy claims and self-report data as measures of medication adherence and to describe baseline characteristics of subjects in the Improving Diabetes Outcomes Study. Methods Multi-ethnic, lower-income, insured adults ( n = 526) in New York City with Type 2 diabetes were enrolled in a randomized, controlled, behavioural intervention study delivered by telephone. Baseline data were examined, including glycated haemoglobin (HbA1c), objective measures of diabetes medication adherence [claims data medication possession ratio (MPR)], and two self-report measures [Morisky Medication-taking Scale and the medication-taking item of the Summary of Diabetes Self-Care Activities (SDSCA)]. Associations of highest tertile HbA1c (≥ 9.3%) with lowest tertile MPR (< 42%) were assessed with logistic regression models adjusting for potential confounders. Subset analyses were performed based on assessment of potential interaction. Results Participants (mean ± sd age 56 ± 7 years) had median (interquartile range) HbA1c 8.6% (8.0–10.0). Correlations of baseline MPR with Morisky score and SDSCA medication-taking item were strongly significant (both ρ = 0.21, P < 0.001). Lowest MPR was significantly ( P = 0.008) associated with highest HbA1c in the group as a whole and among the subset taking two or more oral glucose-lowering agents (OGLA) ( P = 0.002), but not among the subset taking only one ( P = 0.83). Self-report adherence measures were not significantly associated with HbA1c in either the whole group or either subset. Conclusions These results support the validity of MPR as an adherence measure for OGLA among insured diabetes patients with poorly controlled HbA1c, especially those taking two or more OGLA. [ABSTRACT FROM AUTHOR]