A hemoglobin A1c (often just called A1c) is a blood test that measures your average blood sugar over the past two to three months. Unlike a standard blood sugar check, which captures a single moment in time, the A1c gives a longer view of how well your body is managing glucose. It’s one of the primary tools used to screen for, diagnose, and monitor diabetes and prediabetes.
How Glucose Attaches to Hemoglobin
Hemoglobin is the protein inside red blood cells that carries oxygen through your bloodstream. When glucose circulates in your blood, some of it naturally sticks to hemoglobin through a chemical process called glycation. The glucose bonds to a specific spot on hemoglobin’s protein chain, and once attached, it stays there for the life of that red blood cell.
Red blood cells live about 120 days. Because the glucose attachment is permanent and accumulates over time, measuring the percentage of hemoglobin with glucose stuck to it gives a reliable picture of your average blood sugar over roughly the last two to three months. Higher blood sugar means more glucose-coated hemoglobin. The result is reported as a percentage: an A1c of 6%, for example, means that 6% of your hemoglobin has glucose attached to it.
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 diagnosis of diabetes typically requires a second confirmatory test, either a repeat A1c or a different blood sugar measurement. Prediabetes is a warning zone where blood sugar is elevated but not yet at diabetic levels, and lifestyle changes at this stage can often prevent or delay progression to type 2 diabetes.
If you want to translate your A1c into a number that looks more like a daily blood sugar reading, there’s a simple conversion. Multiply your A1c by 28.7, then subtract 46.7. The result is your estimated average glucose (eAG) in mg/dL. So an A1c of 7% corresponds to an average blood sugar of roughly 154 mg/dL, while an A1c of 9% translates to about 212 mg/dL.
What the Test Is Like
The A1c test is a standard blood draw, either from a vein in your arm or a fingerstick. No fasting is required. You can eat and drink normally beforehand, which makes it more convenient than fasting glucose tests. A sample taken from a vein goes to a lab, and results typically come back within a day or two. Some offices use a fingerstick for a rapid in-office result, though this point-of-care version is generally used for monitoring rather than initial diagnosis.
How Often You Need Testing
If you have diabetes and your blood sugar is well controlled, the American Diabetes Association recommends getting an A1c at least twice a year. If you’ve recently changed medications, aren’t meeting your targets, or your doctor is adjusting your treatment plan, you may need it every three months. For people without diabetes, A1c screening is part of routine checkups, particularly after age 45 or earlier if you have risk factors like obesity, family history, or a sedentary lifestyle.
A1c Targets Vary by Age and Health
The standard A1c goal for most adults with diabetes is below 7%, but this isn’t a one-size-fits-all number. Your target depends on your age, overall health, risk of low blood sugar episodes, and how long you’ve had diabetes.
For healthy older adults with a life expectancy of more than 10 years, a target below 7.5% is generally appropriate. Older adults with significant health conditions may aim for 8% or lower, which corresponds to an average blood sugar between 160 and 170 mg/dL. For people with serious comorbidities or cognitive decline, the target may be relaxed further to below 8.5%, with the priority shifting toward avoiding dangerous blood sugar swings and preserving quality of life rather than hitting an aggressive number.
Younger, otherwise healthy people with type 2 diabetes sometimes aim for closer to normal levels, around 6.5%, particularly early in their diagnosis when tight control can reduce long-term complications. The tradeoff is that lower targets increase the risk of hypoglycemia, especially with certain medications.
When A1c Results Can Be Misleading
Because the A1c depends on red blood cells behaving normally, anything that changes how long your red blood cells live or how your hemoglobin is structured can throw off the result.
Conditions that shorten red blood cell lifespan, like hemolytic anemia or recent significant blood loss, will make your A1c appear falsely low. The red blood cells haven’t been around long enough to accumulate a representative amount of glucose, so the number understates your true average blood sugar.
Iron deficiency anemia works in the opposite direction, pushing A1c readings falsely high. This is particularly relevant during late pregnancy, when iron deficiency is common and can elevate A1c even in women without diabetes.
Genetic hemoglobin variants, which are more common in people of African, Mediterranean, and Southeast Asian descent, can also affect accuracy. Variants like hemoglobin S (the sickle cell trait), hemoglobin C, and hemoglobin E may cause results to read higher or lower than reality depending on the specific lab method used. People with sickle cell disease face compounding issues: anemia, faster red blood cell turnover, and frequent transfusions all undermine A1c’s reliability. In these cases, doctors may rely on alternative markers like fructosamine or glycated albumin, which measure glucose attachment to blood proteins with a shorter lifespan and aren’t affected by red blood cell abnormalities.
Chronic kidney disease, especially for patients on dialysis, also tends to make A1c underestimate true blood sugar levels. If you have any of these conditions, your doctor should be interpreting your A1c with those factors in mind or using a different test altogether.
A1c vs. Daily Blood Sugar Monitoring
The A1c and daily blood sugar checks answer different questions. A fingerstick glucose reading or continuous glucose monitor tells you what’s happening right now. Your A1c tells you how things have been going overall. Both matter.
Two people can have the same A1c of 7% but very different daily patterns. One might have steady blood sugar hovering near 154 mg/dL all day. The other might swing between 60 and 250 mg/dL, averaging out to the same number. The A1c can’t distinguish between these two scenarios, which is why it works best alongside daily monitoring rather than as a replacement for it. Those swings carry their own health risks, and catching them requires more granular data than an A1c provides.