Your A1C result reflects how much glucose has attached to hemoglobin, the oxygen-carrying protein inside red blood cells, over the past two to three months. It’s reported as a percentage: the higher your blood sugar has been, the more glucose-coated hemoglobin you have, and the higher your A1C. A normal result is below 5.7%, prediabetes falls between 5.7% and 6.4%, and 6.5% or above indicates diabetes.
What Happens Inside Your Red Blood Cells
Glucose circulating in your bloodstream naturally sticks to hemoglobin through a chemical process called glycation. This isn’t an enzyme-driven reaction your body controls. It happens passively, the way moisture causes metal to rust. The more glucose floating around, the more of it latches onto hemoglobin molecules. Once glucose attaches, it stays there for the life of that red blood cell.
Red blood cells live about 120 days before your body breaks them down and replaces them. At any given moment, your blood contains a mix of brand-new cells, middle-aged cells, and cells nearing the end of their lifespan. An A1C test captures glycation across all of them, giving a rolling average of your blood sugar rather than a single snapshot. That said, the result isn’t weighted equally across the full 120 days. Roughly speaking, the most recent four weeks influence your A1C more heavily than the earlier weeks, because newer red blood cells make up a larger proportion of the sample and have been actively accumulating glucose.
How the Lab Measures It
When your blood sample reaches the lab, the goal is to separate glycated hemoglobin from non-glycated hemoglobin and measure the ratio. Several methods exist, and most labs use one of the following approaches.
- Ion exchange chromatography: The most widely used reference method. It pushes your blood sample through a column that separates hemoglobin types based on their electrical charge. Glycated hemoglobin carries a slightly different charge than regular hemoglobin, so the two separate out and can be measured individually.
- Boronate affinity binding: This technique uses a chemical that selectively binds to the sugar attached to hemoglobin. It pulls glycated hemoglobin out of the sample so it can be quantified on its own.
- Immunoassay: Uses antibodies designed to recognize and latch onto the specific spot where glucose bonds to hemoglobin. This approach is common in automated analyzers.
- Enzymatic methods: Chemical reagents react with the glycated portion of hemoglobin, producing a measurable signal.
All of these methods are standardized to produce comparable results. Correlation between them is high, typically above 0.97, so your A1C from one lab should closely match a result from another.
Converting A1C to Average Blood Sugar
Your A1C percentage can be translated into an estimated average glucose (eAG) using a formula established by the A1C-Derived Average Glucose (ADAG) study. The equation is straightforward:
Estimated average glucose (mg/dL) = (28.7 × A1C) − 46.7
So if your A1C is 7.0%, your estimated average blood sugar over the past two to three months is about 154 mg/dL. At 6.0%, it’s roughly 126 mg/dL. At 5.0%, it’s about 97 mg/dL. Many lab reports now print this estimated average alongside the percentage to make the result more intuitive, since most people are used to seeing blood sugar in mg/dL from finger sticks or continuous glucose monitors.
Outside the United States, A1C is sometimes reported in millimoles per mole (mmol/mol) rather than as a percentage. The conversion formula is: (percentage × 10.93) − 23.50 = mmol/mol. A 6.5% result, for example, equals about 48 mmol/mol.
What the Ranges Mean
The diagnostic thresholds set by the National Institute of Diabetes and Digestive and Kidney Diseases break down like this:
- Below 5.7%: Normal glucose metabolism.
- 5.7% to 6.4%: Prediabetes. The higher you are within this range, the greater your risk of progressing to type 2 diabetes.
- 6.5% or above: Diabetes. A diagnosis typically requires confirmation with a second test unless symptoms are already present.
For people already managing diabetes, A1C targets vary. Many aim for below 7.0%, though your target may differ depending on your age, other health conditions, and how long you’ve had diabetes.
How Often It’s Tested
If you’re being treated for diabetes and haven’t yet reached a stable target, testing every three months makes sense. That interval aligns with the red blood cell lifespan, giving enough time for changes in diet, exercise, or medication to show up in your result. Once your levels are stable, testing every six months is generally sufficient. Labs often enforce a minimum retesting window of 60 to 86 days to prevent unnecessary repeat draws that wouldn’t reflect meaningful change.
When A1C Can Be Misleading
Because A1C depends on hemoglobin and red blood cell turnover, anything that disrupts either one can skew your result. Iron deficiency anemia is one of the most common culprits. When you’re iron deficient, your red blood cells live longer than usual, giving glucose more time to accumulate on hemoglobin. The result: a falsely elevated A1C that makes blood sugar control look worse than it actually is. In pregnant women with iron deficiency anemia, studies have shown A1C dropping from an average of 5.1% to 4.89% after iron treatment, revealing that the higher reading was artificially inflated.
Other conditions that affect red blood cell lifespan can push A1C in either direction. Anything that shortens cell life, like sickle cell disease or significant blood loss, tends to produce a falsely low A1C because cells are replaced before they accumulate much glucose. Anything that extends cell life tends to push it falsely high. Kidney disease, recent blood transfusions, and certain hemoglobin variants can also interfere with results.
If your A1C doesn’t match your daily glucose readings from a meter or continuous monitor, one of these factors may be at play. In those cases, your provider may rely more heavily on direct glucose measurements or use an alternative test called fructosamine, which reflects a shorter window of blood sugar control.