In one sentence
How far the HbA1c your sensor estimates from its average glucose (the glucose management indicator) sits from the HbA1c your laboratory measured, which for some people is a steady gap worth knowing about.
Why it matters
The two numbers should agree and often do not. The glucose management indicator is a line fitted through the relationship between mean sensor glucose and laboratory HbA1c in a diabetes population (Bergenstal 2018), and a person can sit above or below that line for reasons that have nothing to do with their glucose: red cells that live longer or shorter than average, iron deficiency, inherited hemoglobin variants, kidney disease, and the sensor's own bias. In 275 adults with type 1 diabetes, the indicator from one sensor ran 0.58 percentage points above the laboratory HbA1c on average, with the largest gaps (0.9 points) in people whose HbA1c was in the 5.0 to 5.9 percent range, while a second sensor averaged no gap but diverged at both ends of the range (Yoo 2024). The gap matters for one practical reason: once a person knows theirs, they know which of the two numbers to trust for a trend and by how much to correct the other.
What moves it
Anything that changes red cell lifespan or hemoglobin (iron status above all, for a well adult), the sensor model and its bias, and the match between the sensor window and the three months the HbA1c averages, weighted toward the last month.
How to measure it well
A laboratory HbA1c drawn at the end of a sensor month, so that the sensor's window ends where the HbA1c's weight is heaviest; at least 14 days of readings with 70 percent coverage for the indicator; and the same sensor model each time, because the gap is partly the sensor's.
How Vitality Tracker uses it
Computed when a laboratory HbA1c exists within the sensor month and the indicator meets its coverage rule, from the 28 days ending at the draw; shown on the labs dashboard and in the pre-visit report as the gap in mmol/mol with the percentage points in brackets. A gap of 5 mmol/mol or more is marked as worth knowing, in the clinician specification's words, never as a fault in either number; VT keeps the laboratory value, the indicator and any typed value as separate series and never averages them. The gap carries no working optimal; a steady gap across two sensor months is treated as that person's, and the ferritin from the same draw is shown beside it.
How far to trust the number
Calculated from two numbers of different kinds: the HbA1c is a standardized assay, the indicator is an estimate with the sensor's error and a window mismatch inside it. A single gap is a question; the same gap twice is a property.
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
- Yoo JH, Moon SJ, Park CY, Kim JH. Differences between glycated hemoglobin and glucose management indicator in real-time and intermittent scanning continuous glucose monitoring in adults with type 1 diabetes. J Diabetes Sci Technol 2026;20:103–12. doi:10.1177/19322968241262106
- Vitality Tracker, Clinician report content specification (derived indices); Knowledge Base batch 3 (HbA1c, GMI).