| Marker | Gabriel optimal | Typical lab range | Functional range |
|---|---|---|---|
| Hemoglobin A1c | 5 to 5.3 % | 4 to 5.6 | 4.6 to 5.3 |
GMI and A1C side by side
| GMI | Lab A1C | |
|---|---|---|
| Source | Continuous glucose monitor (CGM) | Blood draw or finger-stick sent to a lab |
| What it measures | Glucose in the fluid under your skin, averaged | Share of hemoglobin with glucose attached |
| Time window | The CGM data you choose, ideally 14 days or more | About the past 2 to 3 months |
| Used to diagnose diabetes? | No | Yes, usually confirmed by a second test |
| Affected by red cell lifespan? | No | Yes |
GMI is calculated with a formula: GMI (%) = 3.31 + 0.02392 x your mean CGM glucose in mg/dL. A mean of 150 mg/dL gives a GMI of about 6.9%. Experts suggest at least 10 days of CGM data, and 14 or more is preferred. The name changed from "estimated A1C" after the FDA raised concern that people expected it to match lab A1C.
Why they differ
- Red cells vary from person to person. How long your red cells live, and how readily glucose sticks to them, shift A1C without changing your true glucose.
- Different time windows. A recent illness, steroid course or new low-carb diet changes a 14-day GMI quickly while A1C lags behind.
- Blood conditions. Hemoglobin variants, hemolytic anemia, iron deficiency, blood loss and kidney disease can distort A1C.
- Sensor fluid vs blood. A CGM reads glucose in interstitial fluid, while A1C reflects glucose in the blood over months.
In the 528-person dataset behind GMI, about 19% had values within 0.1 points of each other, 28% differed by 0.5 points or more, and 3% differed by 1 point or more. A person's gap tends to stay stable over time, which can help a clinician set an individual A1C target.
Glucose meters vs lab glucose
Home finger-stick meters are close to lab values but not identical. FDA guidance (September 2020) asks makers of over-the-counter meters to show that 95% of readings fall within 15% of a lab comparison method, and 99% within 20%. So a meter reading of 100 mg/dL could reasonably be a lab value of about 85 to 115.
Clean, dry hands, in-date strips and checking a suspicious reading again help. If your meter, CGM and A1C keep disagreeing, bring all three to your clinician.
What to do with a gap
- Compare like with like. Use a GMI built from at least 14 days of CGM data that overlap the A1C window.
- Look for a reason. Ask whether iron deficiency, recent blood loss, kidney disease or a hemoglobin variant could affect your A1C. A CBC and ferritin are common first checks.
- Track the gap over time. A stable gap matters less than a sudden change.
- Let your clinician decide which number guides treatment. Diagnosis still uses lab tests.
Don't change a diabetes medicine based on a GMI or meter mismatch alone. Contact your clinician, and get care promptly for readings below 70 mg/dL or symptoms of very high blood sugar.
Common questions
Is GMI the same as A1C?
No. GMI estimates A1C from your CGM average glucose. A1C is a lab measure of sugar attached to hemoglobin, and the two often differ by half a point or more.
Which is more accurate, GMI or A1C?
They answer different questions. A1C is the standard for diagnosis and long-term risk, while GMI reflects your actual glucose readings and isn't affected by red cell lifespan. Many clinicians look at both.
Why is my A1C higher than my GMI?
Common reasons include red cells that live longer than average or glycate more readily, iron deficiency, or a recent period of lower glucose that A1C has not caught up with yet.
How accurate are home glucose meters?
FDA guidance asks over-the-counter meters to read within 15% of a lab method 95% of the time. Small differences between your meter and a lab draw are expected.
How many days of CGM data does GMI need?
At least 10 days, and 14 days or more is preferred, according to the paper that defined GMI.
Sources
- Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring (Bergenstal et al., 2018) Diabetes Care
- Self-Monitoring Blood Glucose Test Systems for Over-the-Counter Use (guidance, September 2020) U.S. Food and Drug Administration
- The A1C Test & Diabetes National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
- Hemoglobin A1C (HbA1c) Test MedlinePlus, U.S. National Library of Medicine
This guide is general information, not medical advice. It does not replace a visit with a licensed clinician. Gabriel membership is not health insurance.