A1C is a blood test that measures your average blood sugar over the past two to three months. Unlike a standard glucose test that captures a single moment, A1C reflects the bigger picture of how your body has been handling sugar over time. The result comes as a percentage: the higher the number, the more sugar has been circulating in your blood.
How the A1C Test Works
Your red blood cells contain a protein called hemoglobin, which carries oxygen throughout your body. As blood sugar rises, glucose molecules attach to hemoglobin in a process called glycation. The A1C test measures what percentage of your hemoglobin has glucose stuck to it. If your blood sugar has been running high for weeks or months, more hemoglobin gets coated, and the A1C reading climbs.
The reason this test reflects roughly two to three months of blood sugar is tied to the lifespan of red blood cells. The average red blood cell survives about 115 days in circulation, though individual cells can last anywhere from 70 to 140 days. During that entire lifespan, hemoglobin is continuously picking up glucose. Once a red blood cell dies and gets replaced, the new cell starts fresh. So at any given time, your A1C result represents a weighted average, with more recent weeks contributing slightly more than older ones.
What the Numbers Mean
The American Diabetes Association defines three ranges:
- 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 elevated but not yet at the diabetes threshold. This is a window where lifestyle changes can make a meaningful difference. A result at or above 6.5% on two separate tests is typically enough to confirm a diabetes diagnosis.
Converting A1C to Average Blood Sugar
An A1C percentage can feel abstract, so there’s a formula to translate it into something more concrete: estimated average glucose, or eAG. The conversion is straightforward. Multiply your A1C by 28.7, then subtract 46.7. The result is your estimated average blood sugar in mg/dL.
Here’s what that looks like in practice:
- A1C of 5.7%: average blood sugar around 117 mg/dL
- A1C of 6.5%: average blood sugar around 140 mg/dL
- A1C of 7.0%: average blood sugar around 154 mg/dL
- A1C of 8.0%: average blood sugar around 183 mg/dL
- A1C of 9.0%: average blood sugar around 212 mg/dL
These are averages, not exact figures. Two people with the same A1C can have very different daily patterns. One might have relatively steady blood sugar, while another swings between highs and lows that happen to average out the same way.
How Often You Should Get Tested
If you’re being treated for diabetes and haven’t yet reached a stable target, testing every three months helps track whether your management plan is working. Once your blood sugar is well controlled and consistent, testing every six months is generally sufficient. Your provider may adjust this schedule based on your situation.
One practical advantage of A1C: you don’t need to fast before the test. A fasting glucose test requires you to skip food for eight or more hours, but A1C can be drawn at any time of day regardless of what you’ve eaten. If your provider orders other blood work alongside it (cholesterol panels, for example), those additional tests may still require fasting.
When A1C Results Can Be Misleading
Because the test depends on red blood cells, anything that changes how long those cells survive or how many you have can skew the results. This matters more than most people realize.
Conditions that shorten red blood cell lifespan, like hemolytic anemia or recent significant blood loss, cause red blood cells to turn over faster. Hemoglobin has less time to accumulate glucose, so A1C reads falsely low. You could have genuinely high blood sugar and still get a reassuring-looking number.
Iron deficiency anemia pushes results in the opposite direction, producing a falsely high A1C. This is especially relevant during late pregnancy, when iron deficiency is common. Studies have found that even in pregnant women without diabetes, A1C levels rise due to iron deficiency rather than an actual increase in blood sugar.
Sickle cell disease and other hemoglobin variants also complicate interpretation. The abnormal hemoglobin, combined with chronic anemia and frequent blood transfusions, makes A1C unreliable as a long-term marker. In these situations, providers typically rely on direct glucose monitoring instead of A1C to assess blood sugar control.
If you have any of these conditions, your A1C result still has value, but it needs to be interpreted with context rather than taken at face value.
A1C vs. Daily Glucose Testing
A1C and daily glucose checks tell you different things. Checking blood sugar with a finger stick or continuous glucose monitor shows you what’s happening right now: how a specific meal affected you, whether exercise brought your levels down, or how your blood sugar behaves overnight. A1C can’t capture those details. It gives you the trend line, not the daily ups and downs.
This distinction matters because someone with an A1C of 7.0% might have blood sugar that stays between 130 and 170 mg/dL most of the time, or they might swing from 60 to 280 mg/dL regularly. Both patterns can produce the same A1C, but they represent very different experiences and risks. That’s why many providers use A1C alongside daily monitoring rather than relying on one measure alone.