A1c (also written as HbA1c or hemoglobin A1c) is a blood test that measures your average blood sugar level over the past two to three months. Unlike a standard blood sugar check, which captures a single moment in time, A1c reveals the bigger picture of how your body has been handling glucose. Doctors use it to screen for prediabetes, diagnose type 2 diabetes, and monitor how well diabetes management is working.
How A1c Works in Your Body
The test measures how much glucose has attached itself to hemoglobin, the protein inside red blood cells that carries oxygen. When sugar circulates in your bloodstream, some of it naturally sticks to hemoglobin. The higher your blood sugar runs over time, the more hemoglobin gets coated with glucose.
Once glucose attaches to a red blood cell, it stays there for the cell’s entire life. Red blood cells live roughly 90 to 120 days before your body replaces them. That’s why the A1c reflects a two-to-three-month window rather than a single day. At any given moment, your blood contains a mix of younger and older red blood cells, each carrying a record of the sugar levels they’ve been exposed to. The test reads all of them at once, producing a weighted average that leans slightly more toward recent weeks.
What Your A1c Number Means
A1c results are reported as a percentage. The three diagnostic categories, established by the American Diabetes Association and used across clinical practice, are straightforward:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
A result in the prediabetes range means your blood sugar is higher than normal but hasn’t crossed the diabetes threshold. This is the stage where lifestyle changes, particularly diet and exercise, can sometimes reverse the trend. A reading at or above 6.5% on two separate tests confirms a diabetes diagnosis, unless you already have clear symptoms, in which case one test is sufficient.
For people already managing diabetes, the target is often below 7%, though your doctor may set a more or less aggressive goal depending on your age, health, and risk of low blood sugar episodes.
Converting A1c to Everyday Blood Sugar
Percentages can feel abstract, so it helps to translate your A1c into an estimated average glucose (eAG), the number you’d see on a blood sugar meter. The American Diabetes Association publishes a conversion that maps each A1c percentage to a daily average:
- A1c of 6% = average blood sugar 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
Each 1% increase in A1c corresponds to roughly a 28 to 29 mg/dL jump in average blood sugar. If you’re tracking your blood sugar at home, comparing those daily readings to your A1c can reveal whether you’re catching the full picture. Some people have normal fasting readings but spike sharply after meals, something the A1c will reflect even if your morning finger sticks look fine.
What the Test Is Like
A1c requires a simple blood draw, typically from a vein in your arm, though some point-of-care versions use a finger prick and return results within minutes. No fasting is required. You can eat and drink normally beforehand, which makes it more convenient than a fasting glucose test. Results are usually available within a day or two from a standard lab.
For screening purposes, most adults over 45 are advised to get tested at least once, and sooner if you have risk factors like excess weight, a family history of diabetes, or a history of gestational diabetes. People already diagnosed with diabetes typically get the test every three to six months to track their management over time.
When A1c Results Can Be Misleading
The test is reliable for most people, but certain conditions can push the number artificially high or low. Since A1c depends on hemoglobin and red blood cell lifespan, anything that disrupts either one can skew results.
Conditions that shorten red blood cell survival, like hemolytic anemia or significant blood loss, give glucose less time to accumulate on hemoglobin. The result: a falsely low A1c that makes blood sugar control appear better than it actually is. Kidney failure and dialysis can have a similar effect, which is why alternative markers are sometimes used for those patients.
Iron deficiency anemia pushes results in the opposite direction, producing a falsely high reading. This is especially relevant during late pregnancy, when iron deficiency is common. Even in women without diabetes, A1c can read higher in the third trimester for this reason alone.
Certain genetic hemoglobin variants, such as sickle cell trait or hemoglobin C trait, can also interfere with some testing methods. If you carry one of these variants (more common in people of African, Mediterranean, or Southeast Asian descent), your doctor may need to use a specific lab method designed to account for the variant, or rely on an alternative test like fructosamine, which measures sugar attached to a different blood protein over a shorter window of about two to three weeks.
A1c vs. Other Blood Sugar Tests
A1c isn’t the only way to assess blood sugar. A fasting plasma glucose test measures your blood sugar after at least eight hours without eating. An oral glucose tolerance test checks how your body handles a large dose of sugar over two hours. Both capture a single moment, while A1c captures the trend.
Each test has trade-offs. Fasting glucose can miss people whose blood sugar spikes mainly after meals. The oral glucose tolerance test is more sensitive but takes two hours and requires fasting, making it less practical for routine screening. A1c is the easiest to administer and the hardest to game, since you can’t improve a three-month average by eating well the night before a test. That combination of convenience and reliability is why it’s become the standard tool for both diagnosis and ongoing monitoring.
For people who track blood sugar continuously with a glucose monitor, A1c serves as an independent check on the sensor data. If there’s a significant gap between what your monitor shows and what your A1c says, it’s worth investigating whether one of the accuracy issues described above might explain the discrepancy.