Your CGM numbers
GMI vs. A1C: Why Your CGM Estimate and Lab Result Can Differ
GMI, short for glucose management indicator, is an estimate of A1C worked out from the average glucose on your CGM. It often lands near a lab A1C, but the two measure different things, so in some people they differ by half a point or more.
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The GMI formula
GMI is calculated from your mean (average) CGM glucose:
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL)
For example, a mean glucose of 154 mg/dL gives a GMI of about 7.0%.
The formula comes from research by Bergenstal and colleagues, published in Diabetes Care in 2018. The name GMI replaced “estimated A1C” because the CGM number is not the same measurement as a lab A1C, and a different name makes that clearer.
Why GMI and A1C can differ
They measure different things over different stretches of time.
- A1C is a blood test that reflects how much glucose has attached to hemoglobin in your red blood cells, over roughly the past two to three months. GMI reflects only the days your sensor was worn, often the last 14.
- A1C can be shifted by things that have nothing to do with your glucose, such as conditions that change how long red blood cells live (hemolytic anemia, for example) and some hemoglobin variants (hemoglobinopathies).
- People also differ in how quickly glucose attaches to hemoglobin, so the same average glucose can produce a different A1C in different people.
None of this means either number is wrong. They are two views of the same glucose.
What GMI is useful for
Because it comes from CGM data you already have, GMI can be checked at any time without waiting for a lab draw. Many CGM reports show it next to time in range, which together give you an average and a sense of how widely your glucose moved around it.
When they disagree
A gap between your GMI and your lab A1C is common and usually has an explanation. Your care team can help interpret it and tell you which number they rely on for decisions.
How Dia shows it
Dia’s “Your numbers” screen shows GMI using the published definition (Bergenstal 2018), alongside average glucose and time in range. It describes your data. It does not diagnose, and it does not replace your lab results.
Questions
Is GMI the same as A1C?
No. GMI is an estimate from CGM glucose over a short period. A1C is a lab measurement of glucose attached to hemoglobin over a longer one.
Why is my GMI different from my A1C?
Any of the reasons above can do it. Your last two weeks may differ from the past few months, or your A1C may run higher or lower than your glucose alone would predict. Your care team can look at it alongside your history.
Can I use GMI instead of a lab A1C?
That is a question for your care team. They may use either or both.