Measuring Hemoglobin A1C with Frozen Packed Cell and Frozen Whole Blood Samples in an Epidemiologic Study: The Reasons for Geographic and Racial Differences in Stroke (REGARDS) Study

IntroductionHemoglobin A1c (HbA1c) measurement in epidemiology studies could be increased if reliability of measurements in frozen stored samples was known. In the REasons for Geographic and Racial Differences in Stroke, a longitudinal study of 30,239 Black and White U.S. adults, we investigated reliability of HbA1c measurements for two types of samples stored at -80{degrees}C for up to 14 years. MethodsAmong 917 participants without diabetes, HbA1c was measured in 2017 in frozen packed cells from the first visit (2003-07) and in frozen whole blood samples from the second visit (2013-16). To study reliability, associations between HbA1c and glycemia-related characteristics were examined. ResultsEach 10mg/dL greater fasting glucose was associated with 0.08% (95%CI: 0.05-0.11%) greater HbA1c in frozen packed cells (visit 1) and 0.10% (95%CI: 0.08-0.12%) greater HbA1c in whole blood (visit 2). HbA1c was also similarly higher with both methods with increasing age, gender, systolic blood pressure, body mass index, high-density lipoprotein, triglycerides, C-reactive protein, and hemoglobin. Using both methods, [≤]3.5% were classified with diabetes based on HbA1c [≥]6.5%. ConclusionsIn REGARDS participants without diabetes, HbA1c measurement appeared reliable in frozen packed cells or whole blood under long-term storage, suggesting acceptability for study of the epidemiology of HbA1c.