HbA1c (hemoglobin A1c) is a blood test that measures your average blood sugar over the past two to three months. Unlike a finger stick or fasting glucose test, which captures a single moment in time, the A1c gives a longer view of how well your body is managing blood sugar. It’s used to diagnose diabetes and prediabetes, and to monitor how well treatment is working for people already diagnosed.
How the Test Works
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. This process, called glycation, happens in two steps: glucose first loosely attaches to the hemoglobin molecule, then rearranges into a stable bond that doesn’t come undone. The higher your blood sugar, the more hemoglobin gets coated with glucose. Once glucose attaches, it stays for the life of that red blood cell.
Red blood cells live roughly 90 to 120 days. That’s why the A1c reflects a two-to-three-month window. When your blood is drawn, the lab measures what percentage of your hemoglobin has glucose attached. A higher percentage means your blood sugar has been running higher over that period. It’s essentially a running average baked into your red blood cells.
What the Numbers Mean
The American Diabetes Association uses these cutoffs for diagnosis:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
These numbers can also be translated into an estimated average glucose (eAG), which some people find easier to relate to daily blood sugar readings. For example, an A1c of 7% corresponds to an eAG of about 154 mg/dL. The ADA suggests a target of 7% for most nonpregnant adults with diabetes, though your personal target may differ based on your age, health, and how long you’ve had diabetes.
A result in the prediabetes range is significant. It means your blood sugar is elevated enough to cause gradual damage but not yet high enough for a diabetes diagnosis. Lifestyle changes at this stage, particularly weight loss, dietary shifts, and regular physical activity, can prevent or delay progression to type 2 diabetes.
Why A1c Matters for Complications
The connection between A1c and long-term health outcomes is well established. Lower A1c levels reduce the risk of microvascular complications, which are problems affecting small blood vessels in the eyes, kidneys, and nerves. These are the complications that cause diabetic retinopathy, kidney disease, and peripheral neuropathy. Even modest reductions in A1c can meaningfully lower these risks, which is why the test is central to diabetes management rather than just diagnosis.
No Fasting Required
One practical advantage of the A1c test is that you don’t need to fast beforehand. Because it measures glucose that has accumulated on hemoglobin over months, what you ate for breakfast that morning doesn’t change the result. That said, your doctor may order other blood work at the same appointment (like a cholesterol panel) that does require fasting, so it’s worth asking when you schedule.
How Often You’ll Be Tested
If you have diabetes and haven’t yet reached a stable blood sugar target, testing every three months is standard. This gives enough time for treatment changes to show up in the results, since the test reflects the previous two to three months. Once your blood sugar is consistently well controlled, testing every six months is typically sufficient.
For people without diabetes, A1c is part of routine screening. The frequency depends on your risk factors: age, weight, family history, and whether a previous result fell in the prediabetes range.
When A1c Results Can Be Misleading
The test assumes your red blood cells have a normal lifespan and that your hemoglobin is structurally typical. When either of those assumptions doesn’t hold, the number can be falsely high or low.
Conditions that shorten red blood cell lifespan, such as sickle cell disease or other types of hemolytic anemia, mean hemoglobin spends less time in circulation and has less opportunity to accumulate glucose. The result: an A1c that looks deceptively low, even if blood sugar has actually been elevated. Iron-deficiency anemia can push results in the opposite direction, producing falsely high readings. Chronic kidney disease and heavy bleeding can also skew results.
Hemoglobin variants deserve special mention. People with sickle cell trait, hemoglobin C, or elevated fetal hemoglobin may get inaccurate A1c results depending on the lab method used. Some methods read falsely high, which could lead to unnecessary treatment. Others read falsely low, masking a real problem. For people with sickle cell disease (HbSS, HbCC, or HbSC), the NIDDK advises that A1c should not be used at all. Alternative tests, like fructosamine or continuous glucose monitoring, provide more reliable information in these cases.
Pregnancy can also affect red blood cell turnover and iron levels in ways that make A1c less reliable, which is why gestational diabetes is typically diagnosed with a glucose tolerance test instead.
A1c vs. Daily Blood Sugar Monitoring
The A1c and daily glucose readings tell different stories. A finger-stick glucose reading or continuous glucose monitor shows what’s happening right now: the spike after a meal, the dip during exercise, the overnight trend. The A1c averages all of that into one number. Two people can have the same A1c of 7% but very different daily patterns. One might have steady glucose around 154 mg/dL throughout the day. The other might swing between 80 and 250 mg/dL, averaging out to the same number.
This is why many clinicians look at both. The A1c confirms the overall trend, while daily readings reveal the variability that the A1c can’t capture. Large swings in blood sugar carry their own risks even when the average looks acceptable, so one number doesn’t replace the other.