An A1C test is a blood test that measures your average blood sugar level over the past two to three months. It works by measuring how much glucose has attached to hemoglobin, a protein inside your red blood cells. The result comes back as a percentage: the higher the percentage, the higher your average blood sugar has been. Doctors use it to screen for prediabetes and diabetes, and to monitor how well blood sugar is being managed over time.
How the Test Works
Glucose circulating in your bloodstream naturally sticks to hemoglobin. Once attached, it stays there for the entire life of that red blood cell, which is roughly three months. So when a lab measures the percentage of hemoglobin coated with glucose, it’s essentially reading a running log of your blood sugar over that window. A single high-sugar meal or a stressful week won’t dramatically shift the number the way it would with a fingerstick glucose reading. The A1C reflects the bigger picture.
This is what makes the test so useful compared to a fasting glucose check, which only captures a single moment in time. Two people could have the same fasting number in the morning but very different blood sugar patterns throughout the day. The A1C picks up those differences.
What the Numbers Mean
The CDC uses these thresholds to interpret A1C results:
- Below 5.7%: Normal blood sugar regulation
- 5.7% to 6.4%: Prediabetes
- 6.5% or above: Diabetes
These percentages can also be translated into an estimated average glucose, or eAG, which gives you a number in the same units as a home glucose meter. For example, an A1C of 7% corresponds to an eAG of about 154 mg/dL. This conversion can make the result feel more concrete, especially if you already track your blood sugar daily.
For most nonpregnant adults with diabetes, the American Diabetes Association suggests a target A1C of 7%. But that target isn’t universal. Older adults with other health conditions or limited life expectancy may have a looser goal, since aggressive blood sugar lowering can sometimes cause more harm than benefit in that group. For children, targets may also vary based on the child’s age and the family’s ability to manage a complex treatment routine safely.
No Fasting Required
One of the biggest practical advantages of the A1C test is that you don’t need to fast beforehand. Stress, exercise, and what you ate for breakfast that morning have no meaningful effect on the result. You can walk into a lab at any time of day, have blood drawn, and get an accurate reading. This makes it easier to fit into a routine checkup and removes a common barrier to testing.
Compare that to a fasting plasma glucose test, which requires you to avoid eating for at least eight hours, or an oral glucose tolerance test, which involves drinking a sugary solution and sitting in a lab for two hours. The A1C skips all of that.
When A1C Results Can Be Misleading
The test assumes your red blood cells live a normal lifespan. When that assumption breaks down, the results can be unreliable. Several conditions can push your A1C reading artificially higher or lower than your actual average blood sugar.
Conditions that shorten the life of red blood cells, like sickle cell disease, other hemoglobin variants, or hemolytic anemia, tend to produce falsely low A1C values. Your body is replacing red blood cells faster than normal, so there’s less time for glucose to accumulate on them. Major blood loss has the same effect, flooding your system with younger red blood cells that haven’t had time to collect much glucose.
Iron deficiency anemia works in the opposite direction, pushing A1C results higher than they should be. If you have severe iron deficiency, correcting the anemia before relying on an A1C reading is the more accurate approach. High triglycerides, kidney disease, chronic alcoholism, and even long-term aspirin use have also been reported to falsely elevate A1C results with certain lab methods.
Pregnancy, malaria, chronic infections, and smoking can all influence red blood cell turnover enough to make A1C data unreliable. In these situations, a fasting glucose test or oral glucose tolerance test is typically the better option for diagnosing or monitoring diabetes.
Home A1C Test Kits
You can buy at-home A1C kits that use a finger prick instead of a lab blood draw. The convenience is appealing, but accuracy varies widely between brands. The accepted benchmark is that a home test result should fall within 5% of a lab result in at least 90% of samples.
A University of Florida study involving 219 people with diabetes tested three popular home kits against lab results. One brand, Home Access, met the accuracy benchmark in 82% of samples, which is close but still below the 90% standard. The other two brands performed significantly worse: A1cNow+ met the mark in only 46% of samples, and CoreMedica in just 29%. If you’re using a home kit to track trends between doctor visits, it can offer a rough sense of direction. But for diagnostic decisions or precise monitoring, a lab draw is more reliable.
How Often You’ll Get Tested
If you don’t have diabetes and your results are normal, your doctor will likely recheck every three years or so, depending on your risk factors (family history, weight, age, ethnicity). If you’re in the prediabetes range, testing every one to two years helps track whether lifestyle changes are moving the number in the right direction.
For people with diabetes, the typical schedule is two to four times per year. Someone with stable, well-controlled blood sugar may only need it twice annually, while someone adjusting medications or struggling with control will benefit from more frequent checks. Because the test reflects a two-to-three-month average, testing more often than every three months won’t give you meaningfully new information.