Glucose Management Indicator (GMI) Calculator
Glucose Management Indicator (GMI) Calculator
Calculate GMI from mean CGM glucose, the term that replaced "estimated A1C" for continuous glucose monitoring data.
Glucose Management Indicator
CGM-derived estimateMean CGM glucose 165 mg/dL
Formula
- mean CGM glucose
- the mean sensor glucose over the reporting period, mg/dL
- 3.31, 0.02392
- the intercept and slope from Bergenstal et al.'s regression of CGM mean glucose against laboratory HbA1c
- reporting period
- at least 14 days of CGM data, per international consensus on CGM metrics
Worked example
Mean CGM glucose 165 mg/dL
0.02392 × 165 = 3.9468
3.31 + 3.9468 = 7.3% GMI
Mean glucose to GMI
| Mean CGM glucose (mg/dL) | GMI (%) |
|---|---|
| 100 | 5.7 |
| 120 | 6.2 |
| 150 | 6.9 |
| 165 | 7.3 |
| 180 | 7.6 |
| 200 | 8.1 |
Why GMI is not "estimated A1C"
GMI is calculated from the mean glucose recorded by a continuous glucose monitor, using a regression fitted by Bergenstal and colleagues between CGM mean glucose and a laboratory-measured HbA1c drawn at the same time. It was deliberately named Glucose Management Indicator rather than kept as the earlier informal term “estimated A1C”, because that name implied an accuracy the number does not have — GMI is a distinct, CGM-derived metric, not a stand-in prediction of the laboratory result.
It commonly differs from a same-week laboratory HbA1c by 0.3 percentage points or more in either direction, and a persistent gap of this size is not a fault in either measurement. HbA1c integrates glucose exposure over the full lifespan of the red cell, roughly 90 to 120 days, weighted toward the most recent month, whereas GMI is calculated from a much shorter CGM window, typically 14 days, and reflects only the sensor’s interstitial glucose readings over that time.
A persistent discrepancy beyond the usual scatter is worth a specific look at red cell turnover: anaemia, a haemoglobinopathy, chronic kidney disease, a recent transfusion, or pregnancy can all shift a laboratory HbA1c away from what the person’s actual glycaemia would predict, without affecting GMI at all, since GMI depends only on measured glucose rather than on haemoglobin biology.
Frequently asked questions
What is GMI and how is it different from estimated A1C?
GMI is a CGM-derived glucose management indicator that replaced the older informal term “estimated A1C”. The rename reflects that GMI is calculated purely from CGM mean glucose and is not a precise prediction of a laboratory HbA1c.
Why does my GMI differ from my lab HbA1c?
A gap of 0.3 percentage points or more is common because the two measure different things over different windows — GMI from roughly 14 days of sensor glucose, HbA1c from red cell exposure over 90 to 120 days.
How many days of CGM data do I need for GMI?
At least 14 days, per international consensus on CGM reporting, to give a stable mean glucose.
What causes a persistent gap between GMI and HbA1c?
Usually altered red cell lifespan — anaemia, a haemoglobinopathy, chronic kidney disease, a recent transfusion or pregnancy — which shifts HbA1c independent of true glycaemic control, while GMI is unaffected.
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References
- Bergenstal RM et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care. 2018;41(11):2275–80.
- Battelino T et al. Continuous glucose monitoring and metrics for clinical trials: an international consensus statement. Lancet Diabetes Endocrinol. 2023;11(1):42–57.