A1C is a blood test that measures your average blood sugar over the past two to three months. It’s used both to diagnose diabetes and to track how well it’s being managed. A normal A1C is below 5.7%, prediabetes falls between 5.7% and 6.4%, and diabetes is diagnosed at 6.5% or higher.
How the A1C Test Works
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. This process, called glycation, happens continuously and doesn’t require any enzymes. The more glucose circulating in your blood, the more hemoglobin gets coated with it. The A1C test measures what percentage of your hemoglobin has glucose attached.
Because red blood cells live about 120 days, the test captures a rolling window of your blood sugar levels. It’s not a simple average of those four months, though. The measurement is heavily weighted toward recent weeks: roughly half of what the test reflects comes from the last 30 days, about 40% from days 31 through 90, and only 10% from beyond 90 days. That’s why your most recent eating patterns, medication changes, or lifestyle shifts have the biggest influence on your result.
A1C Ranges: Normal, Prediabetes, and Diabetes
The American Diabetes Association uses these cutoffs:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
A diagnosis typically requires a second confirmatory test, either a repeat A1C or a different blood sugar test. The prediabetes range is significant because it signals that blood sugar is elevated enough to cause early damage but hasn’t crossed the diabetes threshold. Catching it at this stage gives you the widest window for reversing the trend through diet, exercise, and weight loss.
What A1C Numbers Mean in Daily Blood Sugar
A1C is expressed as a percentage, which can feel abstract. To make it more concrete, there’s a formula that converts it to an estimated average glucose (eAG), the kind of number you’d see on a home glucose meter. The formula is: A1C × 28.7 − 46.7 = eAG in mg/dL.
Here’s what that looks like in practice:
- A1C of 6%: average blood sugar around 126 mg/dL
- A1C of 7%: average around 154 mg/dL
- A1C of 8%: average around 183 mg/dL
- A1C of 9%: average around 212 mg/dL
- A1C of 10%: average around 240 mg/dL
Keep in mind these are averages. An A1C of 7% could reflect steady blood sugar near 154 mg/dL, or it could reflect wide swings between highs and lows that happen to average out. That’s one reason your doctor may also look at daily glucose readings or use a continuous glucose monitor alongside A1C.
A1C Targets for People With Diabetes
For most nonpregnant adults with diabetes, the standard target is below 7%. Reaching and maintaining that level significantly reduces the risk of complications affecting the eyes, kidneys, and nerves. Research published in BMJ Open Diabetes Research & Care found that each 1% increase in A1C was associated with an 18% higher rate of major cardiovascular events, including heart attack and stroke. Bringing the number down has a meaningful, measurable effect.
That said, 7% isn’t the right goal for everyone. Some people can safely aim lower, and if you can get below 7% without experiencing frequent low blood sugar episodes, that additional reduction offers additional protection. On the other hand, a less aggressive target of below 8% may be more appropriate if you have a long history of diabetes, are older or frail, have a history of dangerous blood sugar drops, or have advanced heart disease. The American Diabetes Association recommends revisiting targets over time as your health, age, and treatment plan evolve.
How Often You Should Get Tested
Testing frequency depends on where you stand. If your results are normal, your doctor will set a schedule based on your age and risk factors, which could mean every few years. If you have prediabetes, the CDC recommends retesting every one to two years to watch for progression. Once you have a diabetes diagnosis, most people get tested at least twice a year. If your treatment plan recently changed, or your blood sugar has been difficult to control, you may need testing every three months to see whether adjustments are working.
Conditions That Can Skew Results
The A1C test depends on two things: how much glucose is in your blood and how long your red blood cells survive. Anything that disrupts either of those can throw the result off. Several conditions shorten red blood cell lifespan, which means hemoglobin has less time to accumulate glucose and your A1C reads falsely low. These include certain types of anemia, significant kidney disease, and liver failure.
Hemoglobin variants are another factor. The most common are hemoglobin S (sickle cell trait), hemoglobin C, hemoglobin D, and hemoglobin E. Depending on the specific lab method used, these variants can push A1C results falsely high or low. People with sickle cell disease (HbSS) or hemoglobin C disease (HbCC) are particularly affected. If you carry a hemoglobin variant, your doctor may use an alternative lab method or rely on a different blood sugar test, such as fructosamine, which measures a shorter window of glucose control and isn’t affected by hemoglobin type.
Pregnancy, recent blood transfusions, and heavy blood loss can also affect accuracy, since all of these change how long your red blood cells have been circulating. If any of these apply to you and your A1C result seems inconsistent with your daily glucose readings, it’s worth flagging that discrepancy so your care team can choose the most reliable testing approach.