The hemoglobin A1C test is a blood test that measures your average blood sugar level over the past two to three months. Unlike a fingerstick glucose reading that captures a single moment, the A1C gives a longer view of how well your blood sugar has been controlled. It’s used both to diagnose diabetes and prediabetes and to monitor how well treatment is working for people who already have a diagnosis.
How the Test Works
Hemoglobin is a protein inside your red blood cells that carries oxygen. When glucose circulates in your bloodstream, some of it naturally sticks to hemoglobin. The higher your blood sugar, the more glucose coats your hemoglobin. The A1C test measures what percentage of your hemoglobin has glucose attached to it.
The reason this test reflects a two-to-three-month window comes down to red blood cell biology. Red blood cells live for roughly three months, and glucose stays stuck to hemoglobin for the entire lifespan of the cell. So at any given time, your blood contains red blood cells of various ages, each carrying a record of the blood sugar levels they were exposed to. The A1C result is essentially a weighted average of all that accumulated glucose, with more recent weeks contributing slightly more than older ones.
What the Numbers Mean
A1C results are reported as a percentage. The ranges used for diagnosis break down like this:
- Normal: Below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
These percentages can also be translated into an estimated average glucose (eAG), which is the number you’d see on a blood sugar meter. The conversion follows a straightforward formula: multiply the A1C by 28.7, then subtract 46.7. For example, an A1C of 7% corresponds to an estimated average blood sugar of about 154 mg/dL, while an A1C of 9% works out to roughly 212 mg/dL. An A1C of 6% translates to about 126 mg/dL.
For most adults with diabetes, the general target is an A1C below 7%, though your specific goal may be higher or lower depending on your age, other health conditions, and how long you’ve had diabetes. Older adults or people at high risk for low blood sugar episodes sometimes aim for a slightly higher target to avoid dangerous drops.
What to Expect During the Test
The A1C is a simple blood draw, either from a vein in your arm or from a fingerstick in a point-of-care device at your provider’s office. You don’t need to fast beforehand, which makes it more convenient than a fasting glucose test. That said, your provider may bundle the A1C with other bloodwork like a cholesterol panel that does require fasting, so it’s worth confirming ahead of time.
Results are typically available within a day or two from a lab, or within minutes if your provider uses an in-office device.
How Often You Should Be Tested
If you have diabetes and your blood sugar is well controlled and stable, testing twice a year is generally sufficient. If your treatment plan recently changed or your blood sugar isn’t at target, you’ll typically be tested every three months. For people without diabetes, the test is used as a screening tool, often starting at age 35 or earlier if you have risk factors like obesity, a family history of diabetes, or a history of gestational diabetes.
When A1C Results Can Be Misleading
The A1C is considered the gold standard for tracking long-term blood sugar, but it has real blind spots. Because the test depends on hemoglobin and red blood cells behaving normally, anything that disrupts either one can skew results.
Conditions that shorten red blood cell lifespan, like hemolytic anemia or recovery from significant blood loss, cause red blood cells to turn over faster than usual. That means hemoglobin has less time to accumulate glucose, and the A1C reads falsely low. Research shows that when red blood cells live fewer than about 74 days (compared to the typical 120), the underestimation becomes clinically significant. In one study, the shortened lifespan accounted for nearly 15% of the variation in A1C results, enough to mask genuinely elevated blood sugar and lead to under-treatment.
Iron deficiency anemia pushes the number the other direction, producing a falsely high A1C. This is especially relevant during late pregnancy, when iron deficiency is common and can raise A1C results even in people without diabetes. Kidney disease requiring dialysis also tends to make A1C unreliable, generally causing it to underestimate true blood sugar levels.
Certain hemoglobin variants, including sickle cell trait and hemoglobin C trait, can interfere with some testing methods. If you carry one of these variants, your provider may need to use a specific type of A1C assay or rely on alternative markers like fructosamine or glycated albumin, which measure glucose attachment to blood proteins other than hemoglobin and reflect a shorter window of about two to three weeks.
A1C vs. Daily Glucose Monitoring
The A1C and daily glucose checks serve different purposes. A fingerstick or continuous glucose monitor shows you what’s happening right now, which is useful for making immediate decisions about food, activity, or medication timing. The A1C tells you how the overall pattern has looked over months, which is what matters for assessing long-term risk of complications like nerve damage, kidney disease, and vision problems.
One limitation of A1C is that it’s an average, which means it can’t distinguish between someone whose blood sugar is steady at 154 mg/dL and someone who swings between 70 and 240 mg/dL throughout the day. Both people could have the same A1C of 7%, but the person with large swings faces different risks and may need different management. That’s why many providers look at A1C alongside daily glucose data for a more complete picture.