A1C (also written as HbA1c) is a blood test that measures your average blood sugar level over the past two to three months. Unlike a standard blood sugar check that captures a single moment, A1C reveals the bigger picture of how well your body has been managing glucose. It’s used to screen for prediabetes, diagnose type 2 diabetes, and track how well diabetes treatment is working over time.
How the A1C Test Works
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. The more sugar circulating in your blood, the more hemoglobin gets coated. An A1C test measures the percentage of your red blood cells with this sugar coating.
The reason A1C reflects roughly two to three months of blood sugar history comes down to the lifespan of red blood cells. Your body replaces them on a rolling cycle of about 90 to 120 days. So the hemoglobin in your blood at any given time carries a chemical record of glucose exposure stretching back over that window. Research shows it takes about 70 days for hemoglobin to reach a stable level of sugar coating at a given blood glucose level, which is why one test captures such a long stretch of data.
What A1C Numbers Mean
The result comes back as a percentage. The CDC uses these cutoffs:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
You can also translate an A1C percentage into an estimated average glucose (eAG), which may feel more intuitive if you’re used to checking blood sugar at home. The conversion formula is: eAG in mg/dL = 28.7 × A1C − 46.7. So an A1C of 7% corresponds to an average blood sugar of about 154 mg/dL, while an A1C of 6% works out to roughly 126 mg/dL.
Target Levels for People With Diabetes
For most non-pregnant adults with diabetes, the recommended target is below 7%. Reaching that level significantly reduces the risk of diabetes-related complications affecting the eyes, kidneys, and nerves.
That target isn’t universal, though. A less stringent goal of below 8% may be more appropriate for older adults with limited life expectancy or people who experience dangerous drops in blood sugar (hypoglycemia) from aggressive treatment. Younger, otherwise healthy people sometimes aim even lower. The right target depends on your overall health, how long you’ve had diabetes, and what medications you’re taking.
How Often You Should Get Tested
If your blood sugar is well controlled and stable, testing every six months is generally sufficient. If you’re not meeting your target, or your treatment plan has recently changed, testing every three months gives your doctor enough data to adjust course. Clinical guidelines recommend no more than four A1C tests per year.
One practical advantage of the A1C test: it doesn’t require fasting. You can eat and drink normally beforehand, and the blood draw can happen at any time of day. This makes it easier to add to a routine lab visit.
When A1C Results Can Be Misleading
Because the test depends on red blood cells behaving normally, anything that changes how long those cells survive can skew results. Conditions that extend red blood cell lifespan expose hemoglobin to glucose for longer, artificially inflating the number. Iron deficiency anemia is one of the most common causes of a falsely high A1C. Vitamin B-12 and folate deficiency anemias can do the same thing.
The reverse is also true. Conditions that shorten red blood cell lifespan, like hemolytic anemia, significant blood loss, or an enlarged spleen, lead to falsely low readings because the cells don’t live long enough to accumulate a normal amount of sugar coating.
Pregnancy is a notable case. Red blood cells turn over faster during pregnancy (lifespan drops from about 120 days to around 90), and the body ramps up production of new red blood cells. This combination typically pushes A1C readings falsely low through the second trimester. For this reason, A1C is not recommended for diagnosing gestational diabetes.
Inherited hemoglobin variants, such as sickle cell trait or hemoglobin C, can also interfere. People who carry one copy of a variant gene (heterozygous) can usually get accurate results as long as the lab uses an appropriate testing method. But for those with two copies (homozygous), A1C readings are generally unreliable. If you know you carry a hemoglobin variant, your doctor may rely on alternative measures like fructosamine testing, which tracks blood sugar over a shorter, two-to-three-week window without depending on hemoglobin.
A1C vs. Daily Blood Sugar Checks
A1C and fingerstick glucose readings answer different questions. A fingerstick tells you what your blood sugar is right now, which is useful for making immediate decisions about food, exercise, or insulin dosing. A1C tells you how the overall pattern has looked for months, which is what matters for long-term health outcomes and treatment planning.
Neither test alone gives the full picture. Two people can have the same A1C of 7% while experiencing very different daily patterns. One might have steady blood sugar hovering near 154 mg/dL. The other might swing between highs of 300 and lows of 50, averaging out to the same number. That’s why many clinicians look at A1C alongside daily glucose data, especially from continuous glucose monitors, to understand both the average and the variability.