An A1C blood test measures the percentage of your hemoglobin (a protein in red blood cells) that has glucose attached to it. The result reflects your average blood sugar level over the past two to three months, giving a much broader picture than a single finger-stick glucose reading. It’s one of the most common tests used to screen for prediabetes, diagnose type 2 diabetes, and monitor how well blood sugar is being managed over time.
How the A1C Test Works
Glucose circulates in your bloodstream and naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. This process is called glycation. The more glucose in your blood, the more of it attaches. Because red blood cells live for about three months on average, the sugar that stuck to hemoglobin during that window stays measurable. Your A1C result is a percentage representing how much of your hemoglobin is coated with glucose. A higher percentage means your blood sugar has been running higher over the past two to three months.
This is what makes A1C different from a standard blood glucose test, which only captures your level at a single moment. You could have a normal fasting glucose reading one morning but still have an elevated A1C if your blood sugar has been spiking throughout the day or after meals for weeks.
What the Numbers Mean
A1C results fall into three general categories:
- Normal: Below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
A result in the prediabetes range means your blood sugar is higher than normal but not yet at the diabetes threshold. This is a window where lifestyle changes, like increasing physical activity and adjusting your diet, can often prevent or delay progression to type 2 diabetes.
If your result is 6.5% or above, your provider will typically confirm the diagnosis with a second test. That might be a repeat A1C, a fasting blood glucose test, or an oral glucose tolerance test, since providers often use more than one test before diagnosing diabetes.
A1C and Your Daily Blood Sugar
If you check your blood sugar at home with a glucose meter, your A1C might seem like a different language. A useful way to bridge the two is through estimated average glucose (eAG), which converts your A1C percentage into the kind of number you’d see on a meter. The American Diabetes Association provides this conversion:
- A1C 6%: average blood sugar of about 126 mg/dL
- A1C 7%: about 154 mg/dL
- A1C 8%: about 183 mg/dL
- A1C 9%: about 212 mg/dL
- A1C 10%: about 240 mg/dL
Each full percentage point on the A1C scale corresponds to roughly a 29 mg/dL increase in average blood sugar. So if your A1C drops from 8% to 7%, your daily average glucose has come down by about 29 mg/dL, which is a meaningful change in terms of reducing risk for complications like nerve damage, kidney problems, and vision issues.
No Fasting Required
Unlike a fasting blood glucose test, you don’t need to skip a meal before an A1C draw. Because it measures a three-month average rather than a snapshot, what you ate this morning won’t affect 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 checking ahead of time.
How Often You’ll Get Tested
The testing schedule depends on how stable your blood sugar is. If you have diabetes and are meeting your blood sugar goals with a consistent treatment plan, testing every six months is typical. If your treatment has recently changed or you’re having trouble reaching your targets, your provider will likely check every three months. For someone being screened for prediabetes or diabetes without a prior diagnosis, A1C is often part of routine blood work during an annual physical, especially after age 45 or if other risk factors are present.
When A1C Results Can Be Misleading
The test is reliable for most people, but certain conditions can skew results. Anything that changes how long your red blood cells survive or alters the structure of hemoglobin can throw off the reading.
Iron-deficiency anemia, for example, can cause a falsely high A1C because red blood cells live longer than usual when iron is low, giving glucose more time to accumulate on hemoglobin. Chronic kidney disease and heavy bleeding can also distort results. Pregnancy affects red blood cell turnover as well, which is one reason providers may rely on other glucose tests during pregnancy instead.
Hemoglobin variants, which are more common in people of African, Mediterranean, or Southeast Asian descent, present a particular challenge. Conditions like sickle cell trait can cause A1C results to read falsely high or falsely low depending on the lab method used. For people with more severe forms such as sickle cell disease (HbSS) or hemoglobin SC disease, the A1C test is not considered reliable at all, and providers use alternative ways to track blood sugar over time. Certain medications, including some HIV treatments and opioids, can also affect accuracy.
If you have any of these conditions, your provider may use a fructosamine test or continuous glucose monitoring to get a more accurate picture of your blood sugar trends rather than relying on A1C alone.