Author Affiliations: Department of Medicine, University of Alberta, Edmonton, Alberta, Canada.
We appreciate the insightful comments by Dr Onuigbo in regard to our recent study describing the association between hemoglobin A1c (HbA1c) and adverse outcomes in diabetic patients with chronic kidney disease (CKD).1
Although use of a baseline value to define exposure is a commonly used approach in epidemiological research, we agree that glycemic control can change over time in a given patient, and we performed additional, complementary analyses to account for this. First, we used the mean of all HbA1cvalues measured during the initial 6-month exposure period to reclassify HbA1c categories. Second, we considered HbA1c as a time-dependent covariate using all HbA1c values during the study period, prior to the date of each adverse outcome. Our results were similar in all analyses, suggesting that any misclassification resulting from using the first HbA1c value was minimal. Given the design of our study, we cannot conclusively determine whether glycemic control per se affected clinical outcomes or if patients with high initial HbA1c values were resistant to glycemic control measures. We appreciate the comparison to the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC),2 but we note that this cohort was very different than our own, suggesting that their results may not be generalizable to the population we studied.
Shurraw S, Tonelli M. Association Between Glycemic Control and Adverse Outcomes in Diabetic Patients With CKD—Reply. Arch Intern Med. 2012;172(7):597–598. doi:10.1001/archinternmed.2012.227