Abstract P149: The Impact on HbA1c of Adherence Measured Over Multiple Short Intervals: A More Direct Assessment of Biologic Response
Adherence to medications is typically estimated over a 6-12 month period and associated with a mean biologic response over a similar timeframe. Our objective was to examine the relationship between a specific HbA1c measurement and a measure of adherence calculated from medication dispenses over the preceding 90 days among patients who newly initiated metformin therapy.We identified 3,109 persons with type 2 diabetes who initiated metformin as their first ever anti-hyperglycemic drug, analyzing all 9,918 HbA1c measurements that occurred over the next 2 years. We used an adaptation of the proportion of days covered method for assessing medication adherence that corresponded to an approximate 90-day interval preceding an HbA1c measurement, terming the adaptation the “interval-based proportion of days covered” (IB-PDC). To account for multiple observations per patient, we analyzed the association between HbA1c and IB-PDC within the generalized estimating equation (GEE) framework. Analyses were stratified by HbA1c prior to metformin initiation using a cut-point of 8%.
After multivariable adjustment using 0% adherence as the reference category, IB-PDC 1-49% was not associated with HbA1c. However, IB-PDC 50-79% was associated with lower HbA1c values of 0.113 (95% CI: 0.025, 0.202) percentage points among patients with pre-metformin HbA1c <8%, and 0.247 (0.104, 0.390) percentage points among those with HbA1c >8% at metformin initiation. IB-PDC >80% was associated with lower HbA1c of 0.175 (0.093, 0.257) and 0.453 (0.320, 0.586) percentage points for those with HbA1c <8% and >8%, respectively. When re-categorizing IB-PDC into 10% bands using 0-49% as the reference category, HbA1c associated with IB-PDC 50-59% were not significantly different. Among patients with HbA1c < 8% at metformin initiation, all higher levels of IB-PDC were significantly associated with lower HbA1c’s, and IB-PDC >90% was associated with lower HbA1c 0.151 percentage points (95% CI 0.087, 0.216) compared with IB-PDC < 50%. Larger effects were seen among patients with pre-metformin HbA1c > 8%, where IB-PDC 80-89% was associated with HbA1c that was 0.534 (0.403, 0.666) percentage points lower than the reference category.
Using this novel short-interval approach that more closely associates the effects of adherence with the expected biologic response, better medication adherence in obtaining prescriptions was associated with lower HbA1c levels than has been previously reported. The impact of this adherence measure on glycemic control was dependent upon the HbA1c level prior to initiating metformin.
Author Disclosures: G.A. Nichols: B. Research Grant; Significant; Merck & Co, AstraZeneca, Novartis, Boehringer-Ingelheim. A.G. Rosales: None. T.M. Kimes: None. K. Tunceli: A. Employment; Significant; Merck & Co. K. Kurtyka: C. Other Research Support; Significant; Merck & Co. P. Mavros: A. Employment; Significant; Merck & Co. J.F. Steiner: None.
- © 2015 by American Heart Association, Inc.