HbA1c
Blood test · no fasting needed
Also known as: A1c, Hemoglobin A1c, Glycated hemoglobin
- Below the prediabetes range Below 5.7%
- Prediabetes 5.7% to 6.4%
- Diabetes 6.5% and above
Units and reference ranges differ between labs.
Cutoffs: American Diabetes Association, Standards of Care in Diabetes 2026
HbA1c measures how much of the hemoglobin in your red blood cells has sugar attached. It reflects your average blood sugar over roughly the past two to three months. It is one of three tests used to diagnose prediabetes and diabetes.
What it tells you
- Sugar in the blood sticks to hemoglobin, the protein that carries oxygen in red blood cells. The higher your average blood sugar, the more of it is coated.
- Red blood cells live about 120 days, so the result reflects roughly the past two to three months, with recent weeks counting most.
- It changes very little from day to day, far less than a fasting blood sugar, and it does not need an empty stomach.
- A result of 6.5% or higher points to diabetes. Unless blood sugar is clearly very high, a second test confirms the diagnosis.
What can throw it off
- Anything that shortens red blood cell life can make it falsely low. Examples are recent blood loss, a transfusion, red cells breaking down, or treatment with erythropoietin.
- Iron-deficiency anemia and removal of the spleen can make it falsely high.
- Pregnancy lowers it, so it is not used to diagnose diabetes during pregnancy.
- G6PD deficiency, an inherited red blood cell condition, can lower it by about 0.7 to 0.8 points.
- Some hemoglobin variants, such as sickle cell trait, interfere with some lab methods.
- Kidney failure, cirrhosis, and very high doses of vitamin C or E can also distort it.
- When HbA1c and blood sugar readings keep disagreeing, the cause is looked for before either is relied on.
Evidence
Diagnosing diabetes
Across 17 studies, an HbA1c of 6.5% or higher was right about 97% of the time when it said diabetes was present. But it found only about half the diabetes that a glucose tolerance test picked up. A normal result does not rule diabetes out.
Predicting diabetes and heart disease
In a review of 16 studies, fewer than 9% of people at 5.0% to 5.5% developed diabetes within five years. At 6.0% to 6.5%, 25% to 50% did. In a 14-year study of over 11,000 adults without diabetes, higher HbA1c also went with more heart disease. Values below 5.0% were linked to more deaths, not fewer.
Tracking treatment
In large trials, lowering HbA1c reduced damage to the eyes, kidneys and nerves. In type 1 diabetes, tight control cut new eye disease by about three-quarters. In type 2 diabetes, the small-vessel benefit was clear, but fewer heart attacks and strokes were not shown during the trial.
Limits
- It is an average. It does not show blood sugar swings, spikes after meals, or episodes of low blood sugar.
- A normal result does not rule out diabetes. It misses about half the cases a glucose tolerance test finds.
- It does not measure insulin resistance. HbA1c can stay normal for years while the body works harder to keep blood sugar down.
- When a red blood cell or hemoglobin condition distorts it, blood sugar tests or a continuous glucose monitor are used instead.
- It rises slightly with age, but the diagnostic cutoffs are the same at every age.
References
- American Diabetes Association Professional Practice Committee. Diabetes Care. 2026;49(Suppl 1):S27-S49 (Standards of Care, Section 2).
- Kaur G, et al. PLoS One. 2020;15:e0242415.
- Zhang X, et al. Diabetes Care. 2010;33:1665-1673.
- Selvin E, et al. N Engl J Med. 2010;362:800-811.
- The DCCT Research Group. N Engl J Med. 1993;329:977-986.
This page is for education only. It is not medical advice. Decisions about any treatment belong with a physician who knows your history.