A1C to average glucose: the formula and what it means

The equation, term by term

The slope of 28.7 says that each additional whole percentage point of A1C corresponds to roughly 28.7 mg/dL more average glucose; each tenth of a point corresponds to about 2.9 mg/dL. The intercept of 46.7 is subtracted because the fitted line does not pass through the origin — an A1C of zero is not a real state, so the intercept is a property of the regression rather than a meaningful physiological quantity. Nothing in the equation is a threshold or a target; it is a translation between two ways of describing the same underlying glycemia.

Why the term is "estimated"

The word is doing real work. The ADAG study found the relationship between A1C and mean glucose held well at the group level, but individuals at the same A1C had mean glucose values spread meaningfully around the fitted line. Two people can therefore share an A1C and have genuinely different average glucose, and the same two people can have very different glucose variability through the day, which A1C does not capture at all. An estimated average is a summary of a summary.

How this differs from GMI

If your average comes from a continuous glucose monitor rather than a laboratory A1C, there is a different equation for that: the Glucose Management Indicator, GMI = 3.31 + 0.02392 multiplied by mean glucose in mg/dL, published by Bergenstal and colleagues in 2018. That paper introduced the term specifically to stop the CGM figure being called an estimated A1C, because the CGM-derived and laboratory numbers commonly differ in the same person. Using the right equation for the right source of data matters more than it sounds.

Official sources

• Nathan et al., Translating the A1C Assay Into Estimated Average Glucose Values, Diabetes Care 2008
• American Diabetes Association — Standards of Care in Diabetes

Frequently asked questions