Glycosylated hemoglobin, commonly called HbA1c or simply A1C, is a form of hemoglobin with glucose permanently attached to it. It serves as a blood test that 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. An A1C below 5.7% is considered normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
How Glucose Attaches to Hemoglobin
Hemoglobin is the protein inside red blood cells that carries oxygen. As blood sugar circulates through your bloodstream, glucose molecules naturally latch onto hemoglobin in a two-step chemical process called glycation. First, glucose bonds loosely to one end of hemoglobin’s protein chain. Then that unstable bond rearranges into a permanent, stable attachment. The higher your blood sugar runs on a daily basis, the more hemoglobin molecules end up coated with glucose.
Red blood cells live for roughly 120 days before the body breaks them down and replaces them. Older red blood cells have spent more time circulating through glucose-rich blood, so they carry more glycated hemoglobin than younger cells. At any given moment, your blood contains red blood cells of all ages, from brand new to nearly four months old. The A1C test captures a weighted average of glycation across all of these cells, which is why the result reflects approximately two to three months of blood sugar history rather than a single moment in time.
What the Numbers Mean
A1C results are reported as a percentage, representing the fraction of your hemoglobin that has glucose attached. The American Diabetes Association uses three ranges for diagnosis:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
You can also convert A1C into an estimated average glucose (eAG) using the formula: eAG in mg/dL = (28.7 × A1C) minus 46.7. So an A1C of 7% translates to an average blood sugar of roughly 154 mg/dL, while an A1C of 9% corresponds to about 212 mg/dL. Many lab reports now include this estimated average alongside the percentage, making it easier to connect the number to what your glucose meter shows day to day.
Outside the United States, you may see A1C reported in mmol/mol instead of a percentage. This is the international standard set by the IFCC. In those units, a normal range is roughly 20 to 42 mmol/mol, and the diabetes treatment target of less than 7% translates to less than 53 mmol/mol.
Why A1C Matters for Complications
The reason clinicians track A1C so closely is that it directly correlates with the risk of diabetes complications. Every 1% reduction in A1C lowers the risk of diabetic complications by 12% to 43%, depending on the specific complication. Damage to small blood vessels (the kind that feeds the eyes, kidneys, and nerves) is especially sensitive to long-term blood sugar levels. Bringing your A1C down from, say, 9% to 7.5% represents a meaningful reduction in your chances of developing vision problems, kidney disease, or nerve damage over the years.
Target Goals Are Personalized
A blanket target of below 7% applies to most nonpregnant adults with diabetes, per American Diabetes Association guidelines. But the right number for you depends on your overall health. Older adults with limited life expectancy or those who face serious risks from low blood sugar episodes may aim for a less stringent target, such as below 8%. Children with type 1 diabetes typically have a target below 7.5%.
During pregnancy, A1C is less reliable because red blood cells turn over faster, living closer to 90 days instead of the usual 120. That shorter lifespan means less time for glycation, which can make results appear falsely low. Pregnant women with diabetes generally rely more on daily glucose monitoring than on A1C alone.
How Often to Get Tested
If your blood sugar is well controlled and stable, testing every six months is sufficient. If you’re adjusting medications, starting a new treatment plan, or your glucose isn’t at target, testing every three months is standard. Guidelines recommend no more than four A1C tests in a single year. One practical advantage of the test is that it doesn’t require fasting. You can have your blood drawn at any time of day regardless of when you last ate, though your doctor may bundle it with other tests like cholesterol that do require fasting.
When A1C Results Can Be Misleading
Because the test depends on how long red blood cells survive, anything that changes red blood cell lifespan can throw off results. Conditions that slow red blood cell turnover, keeping cells in circulation longer than usual, lead to falsely high A1C readings. Iron deficiency anemia is the most common culprit. Vitamin B12 and folate deficiency anemias have the same effect, as does having no spleen (which normally filters out old red blood cells).
The reverse is also true. Conditions that destroy red blood cells faster or cause blood loss shorten their lifespan, producing falsely low readings. This includes hemolytic anemia, chronic blood loss, an enlarged spleen, and end-stage kidney disease (where chronic anemia accelerates red cell turnover). If you have any of these conditions, your A1C may look better than your actual blood sugar control warrants.
Hemoglobin variants, such as sickle cell hemoglobin (HbS) or hemoglobin C (HbC), create another layer of complexity. People who carry two copies of a variant (homozygous) generally cannot rely on standard A1C testing at all, because the altered hemoglobin structure interferes with the measurement. Those who carry one copy (heterozygous, like sickle cell trait) can still use A1C, but only with specific lab methods designed to handle the variant accurately. If you know you carry a hemoglobin variant, it’s worth confirming that your lab uses an appropriate assay.
Kidney failure also complicates readings through a different mechanism. Elevated waste products in the blood can cause a substance called carbamyl-hemoglobin to form, which some testing methods mistake for glycated hemoglobin, pushing results falsely higher. Yet the shortened red blood cell lifespan in kidney disease simultaneously pushes results lower. These opposing effects make A1C particularly unreliable in advanced kidney disease, where alternative markers of blood sugar control are often preferred.