A1C

Quick definition. A1C (also written HbA1c) is a single blood test that estimates your average blood sugar over the previous two to three months. It is the standard yardstick for diagnosing diabetes and for judging whether a treatment is working.

What A1C Measures

A1C measures the percentage of hemoglobin — the oxygen-carrying protein in red blood cells — that has glucose attached to it. Because glucose binds to hemoglobin in proportion to how much is circulating, and because red blood cells live about three months, the A1C percentage reflects average blood sugar over that window rather than a single moment. This makes it far more stable than a fingerstick reading, which swings with every meal. The terms "A1C" and "HbA1c" refer to the same test.

The Diagnostic Cutoffs

The American Diabetes Association uses three bands to interpret an A1C result:

• Normal: below 5.7%
• Prediabetes: 5.7% to 6.4%
• Diabetes: 6.5% or higher

A diagnosis of diabetes generally requires either two abnormal results or one abnormal result confirmed by a second test, since lab variation and certain conditions can shift the number.

Estimated Average Glucose

A1C can be translated into an estimated average glucose (eAG) in the same mg/dL units a home glucose meter reports. An A1C of 6% corresponds to roughly 126 mg/dL, 7% to about 154 mg/dL, and 8% to about 183 mg/dL — each one-point rise reflecting roughly 28 mg/dL of additional average glucose. This conversion helps patients connect the lab value to the readings they see day to day.

Treatment Targets and How GLP-1s Move the Number

For many non-pregnant adults with type 2 diabetes, the ADA suggests an A1C target below 7%, individualized upward for older patients or those at risk of hypoglycemia. GLP-1 receptor agonists are among the most effective non-insulin agents for lowering A1C. In head-to-head trials, semaglutide (Ozempic) produced A1C reductions in the range of 1.5% to 1.8%, and the dual agonist tirzepatide (Mounjaro) achieved reductions approaching 2.0% to 2.4% at higher doses in the SURPASS program — among the largest seen for an injectable therapy. Several of these agents also reduced cardiovascular events in their cardiovascular outcomes trials.

When A1C Can Mislead

Because A1C depends on red blood cell lifespan, anything that changes that lifespan can distort the result. Anemia, recent blood loss or transfusion, pregnancy, chronic kidney disease, and certain hemoglobin variants can all make A1C read falsely high or low. In these situations clinicians rely on direct glucose testing or a fructosamine test instead. A1C also cannot capture glucose variability — two people with the same A1C may have very different swings between highs and lows.