An A1C test measures the percentage of your hemoglobin (a protein in red blood cells) that has glucose attached to it. It reflects your average blood sugar over roughly the past three months, giving a much broader picture than a single finger-prick reading. A result below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
How the A1C Test Works
Glucose in your bloodstream naturally sticks to hemoglobin, the oxygen-carrying protein inside red blood cells. The higher your blood sugar runs day after day, the more glucose coats that hemoglobin. Because red blood cells live for about three months before your body replaces them, measuring how much glucose is attached gives a reliable average of your blood sugar over that entire window.
This is what makes A1C different from a standard blood sugar check. A fasting glucose test captures a single snapshot. The A1C captures the whole movie, including overnight highs, post-meal spikes, and everything in between.
What the Numbers Mean
The result comes back as a percentage. The three key ranges, established by the National Institute of Diabetes and Digestive and Kidney Diseases, are straightforward:
- Below 5.7%: Normal blood sugar control
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is elevated but not yet in the diabetes range
- 6.5% or higher: Diabetes
A prediabetes result is worth taking seriously. It means your body is already struggling to manage glucose efficiently, but lifestyle changes at this stage (losing a moderate amount of weight, increasing physical activity) can often prevent progression to type 2 diabetes.
Converting A1C to Everyday Blood Sugar
Percentages can feel abstract. An estimated average glucose (eAG) translation makes the number more concrete by converting it to the same units you’d see on a home glucose meter. The conversion, published in Diabetes Care, follows a simple formula: multiply the A1C by 28.7, then subtract 46.7. That gives you an estimated daily average in mg/dL.
Here’s what common A1C levels look like in practice:
- A1C of 5%: Average blood sugar around 97 mg/dL
- A1C of 6%: Around 126 mg/dL
- A1C of 7%: Around 154 mg/dL
- A1C of 8%: Around 183 mg/dL
- A1C of 9%: Around 212 mg/dL
- A1C of 10%: Around 240 mg/dL
These are averages with a range around them. An A1C of 7%, for example, corresponds to a daily average somewhere between 123 and 185 mg/dL for most people. The number doesn’t tell you about swings within the day, only the overall trend.
What to Expect During the Test
The A1C is one of the easiest blood tests you can get. A sample is drawn from your finger or arm, and you don’t need to fast beforehand. What you ate for breakfast won’t change the result, because the test measures glucose that accumulated on your red blood cells over months, not what’s floating in your blood right now. That said, your provider may order other bloodwork at the same visit (cholesterol, for instance) that does require fasting, so it’s worth confirming ahead of time.
How Often You Need Testing
If you have diabetes and your blood sugar is well controlled, the American Diabetes Association recommends getting an A1C test at least twice a year. If your numbers aren’t where they should be, or you’ve recently changed medications or treatment plans, your provider will likely test more frequently, often every three months. For people without diabetes, a single A1C test is typically part of routine screening, especially after age 45 or if you have risk factors like obesity or a family history of diabetes.
When A1C Results Can Be Misleading
The test assumes your red blood cells have a normal lifespan. Anything that changes how long those cells survive can throw off the result, sometimes significantly.
Conditions that shorten red blood cell life, like hemolytic anemia, recent major blood loss, or being on dialysis, cause red blood cells to turn over faster. That means glucose has less time to accumulate on hemoglobin, pulling the A1C reading artificially low. Your actual average blood sugar may be higher than the test suggests.
The opposite happens with iron deficiency anemia. When your body can’t produce new red blood cells efficiently, the existing ones stick around longer and collect more glucose, pushing the A1C reading artificially high. This also occurs in late pregnancy, when iron deficiency is common, even in people without diabetes.
Genetic hemoglobin variants, including sickle cell trait and hemoglobin C, can also interfere with accuracy depending on the lab method used. If you carry one of these traits, your provider may rely on alternative tests like fructosamine or a continuous glucose monitor to track your blood sugar instead. Kidney failure creates a similar problem because it produces a chemically altered form of hemoglobin that some lab tests misread.
None of this means the A1C is unreliable for most people. It’s the standard screening and monitoring tool for good reason. But if your A1C doesn’t match what your daily glucose readings show, one of these factors could explain the gap.