Hemoglobin A1c is a blood test that measures your average blood sugar level over the past two to three months. Instead of capturing a single moment like a finger-stick glucose reading, it reveals the bigger picture of how well your body has been managing blood sugar over time. A result below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
How the Test Works
Hemoglobin is the protein inside red blood cells that carries oxygen. As blood sugar circulates through your body, glucose molecules naturally attach to hemoglobin in a process called glycation. The higher your blood sugar runs, the more glucose coats your hemoglobin, and the higher your A1c percentage climbs.
Red blood cells live for roughly 70 to 120 days before your body replaces them. Because the test measures the glucose buildup on those cells over their entire lifespan, it provides a weighted average of your blood sugar over the previous two to three months. More recent weeks count slightly more, since newer red blood cells are still accumulating glucose while older ones are being cleared out.
What the Numbers Mean
The American Diabetes Association defines three ranges:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
If you want to translate an A1c percentage into a daily blood sugar number you might recognize from a glucose meter, there’s a straightforward formula: multiply the A1c by 28.7 and subtract 46.7. That gives you an estimated average glucose in mg/dL. For example, an A1c of 7% translates to an average blood sugar of about 154 mg/dL. An A1c of 6% works out to roughly 126 mg/dL.
A1c Goals for People With Diabetes
For most nonpregnant adults with diabetes, the 2025 American Diabetes Association guidelines recommend an A1c below 7%. Some people can safely aim lower than that, which may offer additional protection against complications, as long as they aren’t experiencing frequent or severe episodes of low blood sugar.
Goals aren’t one-size-fits-all. Your target depends on how long you’ve had diabetes, your age, other health conditions, whether you have heart disease or advanced complications, and how well you recognize the symptoms of low blood sugar. Older adults or people with limited life expectancy may have a less stringent goal if the risks of aggressive treatment outweigh the benefits. Your care team sets the target based on your full health picture.
How Often You Should Get Tested
If your blood sugar is stable and you’re meeting your treatment goals, testing every six months is generally sufficient. If your treatment plan recently changed, or your blood sugar isn’t well controlled, testing every three months helps track whether adjustments are working. Since the test reflects roughly two to three months of data, testing more frequently than every three months rarely adds useful information.
No fasting is required before an A1c test. It’s a simple blood draw that can happen any time of day, regardless of when you last ate. That said, your doctor may order other tests at the same appointment (like a cholesterol panel) that do require fasting, so it’s worth asking ahead of time.
When A1c Results Can Be Misleading
Because the test depends on red blood cells behaving normally, anything that changes how long those cells survive or how hemoglobin is structured can skew results.
Conditions that shorten red blood cell lifespan, like hemolytic anemia or recovery from significant blood loss, push A1c readings falsely low. The cells don’t live long enough to accumulate a representative amount of glucose. On the other hand, iron deficiency anemia is associated with falsely elevated A1c readings, even in people without diabetes. This is particularly relevant in late pregnancy, where iron deficiency can raise A1c in otherwise healthy individuals.
Genetic hemoglobin variants, including sickle cell trait and hemoglobin C trait, can also interfere with the accuracy of some testing methods. People with sickle cell disease face a combination of anemia, faster red blood cell turnover, and transfusion effects that make A1c unreliable as a long-term marker. Chronic kidney disease, especially for patients on dialysis, tends to cause A1c to underestimate actual blood sugar levels.
If any of these conditions apply to you, your care team may use alternative markers like fructosamine or glycated albumin, which reflect shorter windows of blood sugar control and aren’t tied to red blood cell lifespan.
A1c vs. Daily Glucose Monitoring
A1c and daily glucose checks answer different questions. A finger-stick or continuous glucose monitor tells you what your blood sugar is right now, which helps with immediate decisions about food, activity, and medication timing. A1c tells you how the overall trend has looked for months, which helps evaluate whether your management plan is working.
Neither replaces the other. You can have a normal A1c while still experiencing wide swings between high and low blood sugar throughout the day, because the highs and lows average out. That’s why many clinicians look at both A1c and glucose variability data together to get the full picture. If you’re using a continuous glucose monitor, the “time in range” metric (how many hours per day your blood sugar stays between 70 and 180 mg/dL) adds context that A1c alone can’t provide.