An A1C blood test measures the percentage of your red blood cells that have glucose (sugar) attached to them, giving you 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 what your blood sugar has been doing around the clock, day and night, for roughly 90 days. It’s one of the primary tools used to screen for prediabetes and diabetes and to monitor how well blood sugar is being managed over time.
How the A1C Test Works
When sugar enters your bloodstream, some of it naturally attaches to hemoglobin, a protein inside red blood cells that carries oxygen. Everyone has some sugar stuck to their hemoglobin. The higher your blood sugar levels run, the more hemoglobin gets coated. The A1C test measures what percentage of your hemoglobin is sugar-coated.
The reason this works as a long-term average comes down to the lifespan of red blood cells. Your red blood cells live about three months before your body replaces them. Glucose sticks to hemoglobin for the entire life of the cell, so the test effectively captures a rolling three-month window of blood sugar activity. A blood draw today reflects what your blood sugar has been doing since roughly three months ago.
What the Numbers Mean
The result comes back as a percentage. The American Diabetes Association’s 2025 diagnostic criteria break it down like this:
- Below 5.7%: Normal blood sugar control
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is higher than normal but not yet in the diabetes range
- 6.5% or higher: Diabetes
A diagnosis of diabetes based on A1C typically requires two abnormal test results, either from separate visits or from two different types of blood sugar tests taken at the same time (for example, an A1C combined with a fasting glucose test). The exception is when someone already has obvious symptoms of high blood sugar, in which case a single test can confirm the diagnosis.
A1C and Estimated Average Glucose
A percentage isn’t always intuitive, so labs often convert A1C into an estimated average glucose (eAG) expressed in mg/dL, the same unit you’d see on a home glucose meter. The conversion follows a straightforward formula: multiply the A1C by 28.7, then subtract 46.7. Here’s what that looks like in practice:
- A1C of 6%: average blood sugar around 126 mg/dL
- A1C of 7%: average blood sugar around 154 mg/dL
- A1C of 8%: average blood sugar around 183 mg/dL
- A1C of 9%: average blood sugar around 212 mg/dL
- A1C of 10%: average blood sugar around 240 mg/dL
One important detail: your eAG will almost always be higher than the average of your home glucose readings. That’s because most people check their blood sugar at times when it tends to be lower, like first thing in the morning or before meals. The A1C captures everything, including post-meal spikes and overnight hours when you’re not checking. So if your meter average and your A1C don’t seem to match, that’s the usual explanation.
No Fasting Required
Unlike a fasting glucose test, the A1C doesn’t require you to skip meals or time the blood draw around food. Because it measures glucose that has accumulated on red blood cells over months, what you ate this morning has no meaningful effect on the result. You can have the test done at any time of day, which makes it one of the most convenient blood sugar tests available.
Target Goals Vary by Person
For most adults with diabetes, the general target is an A1C below 7%. But this isn’t one-size-fits-all. Some people aim lower, around 6.5%, if they can reach that level without frequent drops in blood sugar (hypoglycemia) or other side effects. Others may have a higher target, such as below 8%, if they have a history of severe low blood sugar episodes, other serious health conditions, or if tighter control creates more risk than benefit.
For children and adolescents with type 1 diabetes, below 7% is the standard goal, though less stringent targets like 7.5% or 8% may be appropriate for younger kids who can’t recognize symptoms of low blood sugar or who don’t have access to advanced monitoring technology. For young people with type 2 diabetes, the goal is often slightly tighter at below 6.5%, since they tend to have a lower risk of hypoglycemia.
When A1C Results Can Be Misleading
The test relies on red blood cells behaving normally, so anything that changes how long red blood cells live or how hemoglobin functions can throw off the result. Several common conditions affect accuracy:
Iron deficiency anemia can push A1C results falsely higher, making blood sugar control look worse than it actually is. On the other hand, conditions that shorten red blood cell lifespan, like certain types of hemolytic anemia or significant recent blood loss, can make the number falsely low because the glucose-coated cells are being cleared from the body faster than usual.
Sickle cell disease and other hemoglobin variants can interfere with the test in either direction, depending on the specific variant and the lab method used. People with kidney disease on dialysis may also get inaccurate results because of anemia and other factors related to kidney failure. Blood transfusions and treatments that stimulate red blood cell production can similarly distort the reading. In these situations, other measures of blood sugar control, such as tests that track glucose attached to different blood proteins, may give a more reliable picture.
How Often You’ll Get Tested
If you have diabetes and your blood sugar is stable and within your target range, an A1C test twice a year is typical. If your treatment plan has recently changed, or if your blood sugar isn’t well controlled, testing every three months is more common, since each test reflects a new three-month window. For people being screened for prediabetes or diabetes without a prior diagnosis, the test is often part of routine bloodwork during an annual checkup, especially for those with risk factors like excess weight, a family history of diabetes, or age over 45.