In one sentence
An HbA1c-like figure worked out from your sensor's mean glucose over at least fourteen days: the sensor's estimate of what the laboratory would measure, which VT keeps apart from the laboratory's own number and compares with it.
Why it exists
People wanted their sensor to tell them their HbA1c, and the older estimate (eA1c) confused as many as it helped, because the laboratory's result and the estimate often disagreed. In 2018 the US regulator asked for the name to change, and an expert group renamed it the glucose management indicator, built a new conversion from recent trials that used the most accurate sensors available, and set out how to explain the gap between the two numbers rather than hide it (Bergenstal 2018). The gap is the point: a GMI and an HbA1c that differ by a steady amount tell you something about your red blood cells, not about the sensor.
What moves it
Only the mean glucose over the period: everything that moves the mean moves the GMI, in proportion. A short period, low coverage or a week of being unwell moves it for reasons that have nothing to do with your usual glucose, which is why the fourteen-day, 70 percent floor applies.
Why it differs from your lab result
The conversion is a population line; your own red cells may live a little longer or shorter than average, iron status, pregnancy, kidney function and some hemoglobin variants shift the laboratory's number, and the sensor reads tissue fluid, not blood. Two people with the same month of sensor readings can have HbA1c results several mmol/mol apart. For many people the gap is stable from year to year, and once known it makes both numbers easier to read.
How Vitality Tracker uses it
Computed from any period of at least 14 days with at least 70 percent coverage; the Baseline Month's GMI is the first one you see. VT keeps it as its own series beside the laboratory HbA1c and any typed value, never blends them, and shows the GMI-versus-HbA1c gap on the labs dashboard and in the pre-visit report when both exist within the same window. The Vitality Score's glucose area never reads the GMI: its HbA1c slot takes your laboratory HbA1c or the sensor's estimated A1c, whichever is more recent.
How far to trust the number
Estimate: as good as the mean it is built from, converted by a line fitted to other people. It is not a diagnosis of anything and does not replace a laboratory test; its job is to be compared with one.
Sources
- Bergenstal RM, et al. Glucose management indicator (GMI): a new term for estimating A1C from continuous glucose monitoring. Diabetes Care 2018;41:2275–80. doi:10.2337/dc18-1581
- Battelino T, et al. Diabetes Care 2019;42:1593–603. doi:10.2337/dci19-0028.