Overview
What A1C Captures—and What It Cannot
A1C is a backward looking measure of glucose exposure.
A1C is a backward-looking measure of glucose exposure. Glucose binds nonenzymatically and irreversibly to hemoglobin within circulating erythrocytes. Because erythrocytes live for about 120 days, the result reflects average glycemia over approximately the preceding 8–12 weeks. It is not an equal three-month average. Recent glucose exposure, especially the prior 30 days, contributes more heavily than older exposure. A patient whose glucose improved dramatically last week may still have an elevated A1C because the older glucose exposure remains represented in circulating cells. A1C answers, “What has the average glucose exposure been?” It does not answer, “How often is this patient hypoglycemic?” or “How much are glucose values fluctuating?” Two patients may both have an A1C of 7%, approximately an estimated average glucose of 154 mg/dL, yet have very different safety profiles: - One may remain near target most of the day. - Another may alternate between repeated values in the 50s and marked post-meal hyperglycemia. The average can look acceptable while the patient is having dangerous lows. Continuous glucose monitoring (CGM) or structured fingerstick data exposes that pattern. A1C also does...