GMI vs Laboratory HbA1c: Why Your CGM Estimate and Blood Test Can Differ
Continuous glucose monitoring (CGM) reports include a Glucose Management Indicator (GMI) expressed as a percentage. Learn the exact formula behind GMI and why a difference of 0.3% to 0.5% or more between GMI and lab HbA1c is common.
The Bergenstal (2018) GMI Formula vs the 2008 ADAG eAG Formula
In 2018, Bergenstal and colleagues introduced the term Glucose Management Indicator (GMI) to replace the older label "estimated A1C (eA1C)" on CGM reports. The published regression equation is GMI (%) = 3.31 + 0.02392 × [mean CGM glucose in mg/dL].
Notice that this formula has a different slope and intercept than the 2008 ADAG equation (eAG = 28.7 × A1C − 46.7). For example, at a mean glucose of 150 mg/dL, GMI equals 6.898% (6.9%), whereas the inverted ADAG equation gives 6.85%. At 200 mg/dL, GMI equals 8.09% (8.1%), whereas ADAG gives 8.60%.
| Mean CGM Glucose (mg/dL) | Mean CGM Glucose (mmol/L) | GMI (%) — Bergenstal 2018 | GMI (mmol/mol) — IFCC equiv. |
|---|---|---|---|
| 100 mg/dL | 5.6 mmol/L | 5.7% | 39 mmol/mol |
| 125 mg/dL | 6.9 mmol/L | 6.3% | 45 mmol/mol |
| 150 mg/dL | 8.3 mmol/L | 6.9% | 52 mmol/mol |
| 175 mg/dL | 9.7 mmol/L | 7.5% | 58 mmol/mol |
| 200 mg/dL | 11.1 mmol/L | 8.1% | 65 mmol/mol |
| 225 mg/dL | 12.5 mmol/L | 8.7% | 72 mmol/mol |
Why GMI and Laboratory HbA1c Can Diverge in the Same Person
In the clinical trials used to derive GMI, roughly half of participants had a laboratory HbA1c that differed from their GMI by at least 0.3 percentage points, and over 25% differed by 0.5 percentage points or more.
Key reasons include: (1) Time window mismatch — a 14-day CGM summary captures the past 2 weeks, while lab HbA1c reflects 8–12 weeks of glycation; (2) Red blood cell lifespan — faster or slower red cell turnover changes how long hemoglobin is exposed to glucose; and (3) Sensor calibration and wear gaps.
Limitations and When to Seek Medical Care
- GMI is intended for continuous glucose monitoring datasets spanning at least 10 to 14 days with >=70% active sensor data.
- Neither GMI nor home calculators replace diagnostic venous HbA1c testing ordered by your clinician.
Related Calculators
GMI Calculator (Glucose Management Indicator)
Calculate your Glucose Management Indicator (GMI) in both NGSP-aligned % and IFCC-equivalent mmol/mol from your continuous glucose monitor (CGM) mean glucose using the published Bergenstal et al. (2018) formula.
A1C to eAG Calculator
Translate laboratory NGSP HbA1c (%) into estimated average glucose (eAG) in both mg/dL and mmol/L using the Nathan et al. (2008) ADAG linear regression equation.
HbA1c Unit Converter (mmol/mol ↔ %)
Convert laboratory HbA1c results between IFCC standard units (mmol/mol, used in the UK, Europe, and Australia) and NGSP/DCCT percentage units (%, used in the US and Canada) using the published NGSP/IFCC master equation.
Frequently Asked Questions
If my GMI is 6.8% and my lab A1C is 7.3%, is my sensor broken?
Not necessarily. Biological variation in red blood cell glycation and lifespan frequently produces a steady personal offset between interstitial GMI and venous HbA1c.
Sources and Review Information
- [S11] Bergenstal RM, Beck RW, Close KL, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care. 2018;41(11):2275-2280. Reference Source
- [S9] Nathan DM, Kuenen J, Borg R, et al. Translating the A1C assay into estimated average glucose values (ADAG Study). Diabetes Care. 2008;31(8):1473-1478. Reference Source
Prepared by: GlucoBP Editorial & Engineering Team · Verification status: Formula verified against published reference; clinical review pending · Last updated: 2026-11-15