December 17, 2001
A1c (Glycohemoglobin, HgbA1c)
Question from Honolulu, Hawaii, USA:
I was told that the A1c is the long term measure of hyperglycemia and that any blood glucose above 120 mg/dl [mmol/L] triggers the A1c to be above normal. Does it not then hypothesize that if daily blood glucose readings on average are considerably within the hypo-glycemic range that this would thereby balance the fewer hyper- glycemic daily blood glucose levels?
Hemoglobin A1c is an integrated average of blood glucose levels over the life of the red blood cell — approximately 120 days. However, in fact, the more recent blood glucose levels can have more influence than older ones so it is likely a better average of the past 30-60 days or so. Hypoglycemic values would tend to balance hyperglycemic values if they occurred with equal frequency and duration — but the hypos tend to be more transient compared to the number of hyperglycemic episodes because of the relative difficulty of providing adequate insulin — especially in the postprandial time period. Nevertheless, A1c is the best objective long term measure of glucose control, but cannot be used for individual or daily insulin dose decisions since it is a time-average of blood glucose levels.