An A1C blood test measures the percentage of your red blood cells that have glucose (sugar) attached to them, giving a picture of your average blood sugar over the past two to three months. Unlike a standard blood sugar check that captures a single moment, the A1C reflects how your body has been handling glucose day after day, week after week. It’s the primary tool used to diagnose prediabetes and type 2 diabetes, and to monitor how well blood sugar is being managed over time.
How the Test Works
Your red blood cells contain a protein called hemoglobin, which carries oxygen through your bloodstream. When glucose circulates in your blood, some of it naturally attaches to hemoglobin in a chemical reaction called glycation. Once glucose binds, it stays attached for the life of that red blood cell. The higher your blood sugar levels have been, and the longer they’ve stayed elevated, the greater the percentage of hemoglobin with glucose stuck to it.
Red blood cells live about 90 to 120 days before your body replaces them. Because of this turnover cycle, an A1C result represents a weighted average of your blood sugar over roughly the previous two to three months, with the most recent weeks contributing more heavily to the number. This is why the test is sometimes called the “gold standard” for evaluating long-term blood sugar control.
What the Numbers Mean
A1C results are reported as a percentage. The American Diabetes Association’s 2025 standards define three ranges:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
A result in the prediabetes range means your blood sugar has been running higher than normal but hasn’t crossed the diabetes threshold. About 1 in 3 American adults falls into this category, and many don’t know it. At this stage, lifestyle changes like increased physical activity and dietary adjustments can often bring numbers back down.
If your result is 6.5% or above, your doctor will typically confirm the diagnosis with a second test on a different day, unless you already have classic symptoms of high blood sugar like frequent urination, increased thirst, or unexplained weight loss.
Converting A1C to Average Blood Sugar
Because A1C is expressed as a percentage, it can feel abstract. You can convert it to an estimated average glucose (eAG) in the same units a home glucose meter uses. The formula is: multiply your A1C by 28.7, then subtract 46.7. The result is your estimated average blood sugar in mg/dL.
To put that in practical terms: an A1C of 5.7% (the start of prediabetes) corresponds to an average blood sugar of about 117 mg/dL. An A1C of 6.5% (the diabetes threshold) works out to roughly 140 mg/dL. And an A1C of 7%, a common treatment target for people with diabetes, translates to an average of about 154 mg/dL. Seeing your A1C as a daily blood sugar number can make it easier to connect lab results with what’s actually happening in your body.
No Fasting Required
One of the practical advantages of an A1C test is that you don’t need to fast beforehand. You can eat and drink normally before the blood draw, which makes scheduling simpler compared to a fasting glucose test. The sample is usually taken from a vein in your arm during a routine lab visit, though some clinics use a quick finger-stick version that returns results in minutes.
Most people with diabetes get an A1C test at least twice a year. If you’re adjusting medications or your blood sugar hasn’t been stable, your doctor may check it every three months. For people without a diabetes diagnosis, the test is commonly included in routine screening, especially after age 45 or if you have risk factors like excess weight, a family history of diabetes, or a sedentary lifestyle.
Conditions That Can Skew Results
The A1C test assumes your red blood cells have a normal lifespan. When that assumption doesn’t hold, results can be misleading. Any condition that shortens red blood cell survival, like hemolytic anemia or significant blood loss, will push your A1C reading falsely low because the cells haven’t been exposed to glucose as long. A shorter lifespan means less time for glucose to accumulate on hemoglobin, so the number looks better than your actual blood sugar control warrants.
Iron deficiency anemia does the opposite, producing falsely elevated A1C results. This is especially relevant during late pregnancy, when iron deficiency is common and can raise A1C even in women without diabetes. Iron replacement therapy has been shown to lower A1C readings in both diabetic and non-diabetic individuals, not because blood sugar changes but because the measurement itself becomes more accurate once iron levels normalize.
Certain inherited hemoglobin variants, particularly sickle cell trait and hemoglobin C trait, can also interfere with results depending on the laboratory method used. People with chronic kidney disease, especially those on dialysis, may get unreliable A1C readings as well because of changes in red blood cell turnover and chemical modifications to hemoglobin. In these situations, doctors sometimes use alternative markers like fructosamine or glycated albumin to get a clearer picture of blood sugar control.
What Your A1C Can and Can’t Tell You
The A1C is excellent at revealing patterns. It tells you whether your blood sugar has been consistently elevated, gradually improving, or well controlled over a span of weeks. This makes it far more useful than a single glucose reading, which can swing based on what you ate that morning, how much you slept, or whether you exercised the day before.
What the A1C doesn’t show is variability. Two people can have the same A1C of 7% while experiencing very different daily patterns. One might have steady blood sugar hovering around 154 mg/dL, while the other swings between lows of 60 and highs of 250 that average out to the same number. For this reason, people managing diabetes often use the A1C alongside daily glucose monitoring to get both the big picture and the moment-to-moment detail. The A1C captures the trend; daily checks capture the swings.