Most people in the clinical lab and among the Diabetes population are aware that there is not a good correlation between a blood glucose level and a hemoglobin A1C (HbA1c) level. This only makes sense. A blood glucose is essentially a snapshot of what the glucose concentration is in your body at that particular point in time when the blood sample is collected. HbA1c is a measure of the percent of your hemoglobin that has glucose attached to it. The longer your glucose is high and the higher it is, the more glucose will be attached to hemoglobin and the higher your percent HbA1c. The hemoglobin with glucose attached stays around for the life of the red blood cell that holds it, 120 days. Therefore, a HbA1c level is an indication of what your blood glucose has been averaging for the last 4 months.
That’s where an estimated average glucose (eAG) comes in. eAG is a value that’s calculated from the HbA1c, so it is also an indicator of what your blood glucose has averaged over the last 120 days rather than being a snapshot of your current blood glucose. Like HbA1c, its utility lies in that a person may not have been in control of their glucose for the last 4 months, but they are careful the day they come in to have their glucose checked. Their snapshot blood glucose may be 130 mg/dL or close to the normal range, but their eAG would still be 200 mg/dL or more, indicating what they’ve been averaging the last 4 months.
The formula for calculating eAG was developed by Nathan et al (1). Their study in which they derived it is impressive. They collected roughly 2700 separate glucose measurements on each of 507 study subjects. The study cohort contained 268 persons with type 1 diabetes, 159 persons with type 2 diabetes and 80 normal controls. All participants in their study had their glucose under control for the 3 month run of the study. With so many individual measurements on each study subject, the authors were able to determine an average glucose for each subject and correlate it with the subject’s HbA1c. The formula they derived is:
eAG = 28.7 X HgbA1C – 46.7 (for US units of mg/dL)
Interestingly, HbA1c gives you essentially the same information as eAG: an indication of what a person’s average glucose has run over the last 120 days. The difference is this: most people monitoring blood glucose know what a glucose value means and when their glucose value is too high. Thus an eAG of 250 mg/dL may make more sense to them than an HbA1c of 10.3 %. They now know that they have been averaging a glucose of 250 mg/dL, even if today’s glucose was 126 mg/dL. Whether it’s true or not that eAG is easier to understand than HbA1c, HbA1c has become widely used and eAG has not. Despite that, it does have the potential to be a useful calculation.
1. Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine JR. Translating the A1C assay into estimated average glucose values. Diabetes Care, 31(8):1473-1478. 2008.
-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.