The Effect of Age and Comorbidities on the Association Between the Medicare STAR Oral Antihyperglycemic Adherence Metric and Glycemic Control

BACKGROUND: The Medicare STAR program for Medicare Advantage Plans that include drug benefits provides monetary incentives for health plans to achieve good adherence to oral antihyperglycemic (OAH) agents but does not account for differential case mix that could affect the ability of health plans to achieve the required quality metrics. OBJECTIVE: To determine whether OAH adherence varies by age and comorbidities among patients aged 65 years or older and the extent to which adherence affects glycemic control across age and comorbidity strata. METHODS: We studied 54,480 patients with diabetes aged > 65 years from the Colorado, Northwest, and Northern California regions of Kaiser Permanente who received OAH agents but not insulin in 2010. We calculated adherence using the proportion of days covered (PDC) method. Per the STAR program, hemoglobin A1c < 8% defined good glycemic control. We also defined poor control as A1c > 9%. We used modified Poisson regression to identify predictors of adherence and to determine its effects on A1c across age and comorbidity strata, adjusting for sociodemographics and medication-related variables. RESULTS: The risk of being adherent to OAH declined moderately with an increasing number of comorbidities (risk ratio [RR] = 0.99, 95% CI = 0.98-1.00 for 1 comorbidity and RR = 0.90, 95% CI = 0.88-0.91 for 4 or more comorbidities). Adherence to OAH agents was associated with a 0%-3% increased risk of A1c < 8% across age and comorbidity categories, as well as a large decreased risk (RR = 0.55-0.73) of A1c > 9% for patients aged < 80 years or with < 3 comorbidities. CONCLUSIONS: Among patients with diabetes aged > 65 years, having multiple comorbidities affects adherence. Adherence reduces the risk of poor A1c control among patients aged 65-79 years or with 2 or fewer comorbidities. Our results suggest that health plan case mix minimally influenced the Medicare STAR OAH adherence metric, but it may affect glycemic control quality measures, especially if a HEDIS-like measure of poor control were adopted.