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A1c, fasting glucose and GMI
Three ways to describe blood sugar. They measure different things, so they do not always agree.
The three numbers
- A1c: glucose attached to hemoglobin, reflecting about 3 months. No fasting needed (NIDDK).
- Fasting plasma glucose and the oral glucose tolerance test (OGTT): glucose in the blood at the time of the test. Both need at least 8 hours of fasting (NIDDK).
- GMI: an A1c-like number calculated from the mean glucose of continuous glucose monitor (CGM) data (Bergenstal 2018).
All can be used for diagnosis, except GMI
The ADA criteria allow A1c, fasting glucose, the OGTT or, with symptoms, a random glucose to diagnose diabetes (ADA Standards 2026, sec. 2). GMI is a tool for people who already use a CGM, not a diagnostic test (Bergenstal 2018).
The cut points side by side
ADA diagnostic thresholds for each test (ADA Standards 2026, sec. 2). A diagnosis needs a confirming test unless symptoms are clear (NIDDK).
| Test | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| A1c | Below 5.7% | 5.7% to 6.4% | 6.5% or higher |
| Fasting plasma glucose | Below 100 mg/dL | 100 to 125 mg/dL | 126 mg/dL or higher |
| 2-hour OGTT glucose | Below 140 mg/dL | 140 to 199 mg/dL | 200 mg/dL or higher |
There is no formula that turns a single fasting glucose into an A1c. The A1c reflects all day and night over about 3 months; a fasting result is one morning. The A1c converter works with average glucose, not fasting glucose.
Why they can disagree
- A glucose test can show diabetes when an A1c does not, and the reverse. That is one reason tests are repeated before a diagnosis (NIDDK).
- Glucose rises and falls with meals, exercise, illness and stress. A1c is less affected by these short-term changes (NIDDK).
- A1c depends on red blood cells. Conditions that change them can make A1c misleading. See when A1c can mislead.
- GMI and a lab A1c from the same person can differ (Bergenstal 2018).
A high A1c with a normal fasting glucose? See A1c high, glucose normal.
Where eAG fits
Estimated average glucose (eAG) is the A1c translated into mg/dL with the ADAG formula (Nathan 2008). It is still an A1c, just in glucose units. It will not match daily meter readings, because it is a long-term average (NIDDK).
Tools: A1c converter · GMI calculator · GMI vs A1c chart.
Sources
- National Institute of Diabetes and Digestive and Kidney Diseases (NIH). The A1C Test & Diabetes. www.niddk.nih.gov/health-information/diagnostic-tests/a1c-test
- American Diabetes Association Professional Practice Committee. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes 2026. Diabetes Care. 2026;49(Suppl 1). doi.org/10.2337/dc26-S002
- 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. doi.org/10.2337/dc18-1581
- Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ; A1c-Derived Average Glucose (ADAG) Study Group. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473-1478. doi.org/10.2337/dc08-0545
Written and maintained by the controla1clevels.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-10-26. Spotted an error? See About.