Hemoglobin A1C is a blood test that measures your average blood sugar over the past two to three months. Instead of capturing a single moment like a finger-stick glucose reading, it reveals the bigger picture of how well your body has been managing blood sugar over time. The result comes as a percentage: the higher the number, the higher your average blood sugar has been.
How the Test Works
Hemoglobin is a protein inside your red blood cells that carries oxygen. When glucose circulates in your bloodstream, some of it naturally sticks to hemoglobin. Once glucose attaches, it stays there for the entire life of that red blood cell, which is roughly 90 to 120 days. The A1C test measures what percentage of your hemoglobin has glucose stuck to it.
When blood sugar runs higher over weeks and months, more glucose coats the hemoglobin, and the A1C percentage rises. When blood sugar stays in a healthy range, less glucose attaches, and the percentage stays low. Because red blood cells are constantly being replaced, the test naturally reflects a rolling average weighted toward the most recent weeks.
What the Numbers Mean
The American Diabetes Association uses these A1C thresholds for adults who are not pregnant:
- Below 5.7%: Normal blood sugar control
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is higher than normal but not yet in the diabetes range
- 6.5% or higher: Diabetes
A result in the prediabetes range is a signal that your body is starting to struggle with blood sugar regulation. It doesn’t mean diabetes is inevitable. Lifestyle changes at this stage, particularly losing a modest amount of weight and increasing physical activity, can slow or reverse the trend.
Translating A1C to Everyday Blood Sugar
Percentages can feel abstract, so there’s a formula that converts your A1C into an estimated average glucose (eAG) in the same units you’d see on a glucose meter. The conversion is: multiply your A1C by 28.7, then subtract 46.7. That gives you an estimated average in mg/dL.
Here’s what that looks like in practice:
- A1C of 5.7%: Estimated average glucose of about 117 mg/dL
- A1C of 6.5%: Estimated average glucose of about 140 mg/dL
- A1C of 7.0%: Estimated average glucose of about 154 mg/dL
- A1C of 8.0%: Estimated average glucose of about 183 mg/dL
These numbers help connect the A1C result to something more concrete. If you check your blood sugar at home, your meter readings should roughly align with your eAG, though day-to-day highs and lows will vary around that average.
How Often You Need the Test
If you have diabetes and your blood sugar is well controlled, you’ll typically get tested at least twice a year. If you’ve recently changed medications or your numbers aren’t where they should be, testing every three months is more common. Your doctor may also order an A1C as part of routine bloodwork if you have risk factors for diabetes, such as being overweight, having a family history, or being over 45.
One practical advantage of the A1C test: you don’t need to fast beforehand. You can eat and drink normally before your blood draw, which makes it easier to schedule. Your doctor may run other tests at the same time (like a cholesterol panel) that do require fasting, so it’s worth confirming ahead of time.
When the Test Can Be Less Accurate
Because A1C depends on red blood cells behaving normally, anything that changes how long your red blood cells live or how hemoglobin is structured can throw off the result.
Conditions that shorten the lifespan of red blood cells, such as hemolytic anemia or recent significant blood loss, cause A1C to read falsely low. The red blood cells haven’t been around long enough to accumulate a representative amount of glucose. On the other hand, iron deficiency anemia can push A1C readings falsely high. This is especially relevant during late pregnancy, when iron deficiency is common.
People with sickle cell disease or sickle cell trait carry a different form of hemoglobin that can interfere with certain A1C testing methods. Chronic kidney disease also complicates things: the combination of altered hemoglobin, anemia, and other factors can make A1C unreliable in either direction for people on dialysis. In these situations, doctors may rely on alternative markers of blood sugar control, such as a test called glycated albumin, or more frequent glucose monitoring.
If you have any of these conditions, your A1C should be interpreted with extra context rather than taken at face value.
A1C Goals for People With Diabetes
For most adults with diabetes, the general target is an A1C below 7%. This threshold is associated with significantly lower risk of complications affecting the eyes, kidneys, and nerves. But the right target varies from person to person. Younger adults who were recently diagnosed may aim for something closer to 6.5%, while older adults or those with other serious health conditions might have a more relaxed goal closer to 8% to reduce the risk of blood sugar dropping dangerously low.
Dropping your A1C by even half a percentage point can meaningfully reduce your risk of long-term complications. Small, consistent changes in diet, exercise, and medication adherence tend to move the number more reliably than dramatic short-term efforts. Since the test reflects a two-to-three-month window, it rewards sustained habits over quick fixes.