A hemoglobin A1C test measures the percentage of your red blood cells that have glucose (sugar) attached to them, giving a picture of your average blood sugar over the past two to three months. Unlike a standard blood sugar test that captures a single moment, the A1C acts as a long-term scorecard. A normal result is below 5.7%, prediabetes falls between 5.7% and 6.4%, and diabetes is diagnosed at 6.5% or higher.
How the Test Works
Hemoglobin is the protein inside red blood cells that carries oxygen. As blood sugar circulates through your body, some of that glucose naturally sticks to hemoglobin. The higher your blood sugar runs on a day-to-day basis, the more hemoglobin gets coated with glucose. Once glucose attaches, it stays for the entire life of that red blood cell.
Red blood cells live roughly 90 to 120 days before your body replaces them. That turnover is what makes the A1C so useful: when a lab measures the percentage of sugar-coated hemoglobin in your sample, it’s effectively averaging your blood sugar across two to three months of red blood cell generations. A single high-sugar meal won’t budge the number, and a single good day won’t save it. The result reflects what’s been happening consistently.
What the Numbers Mean
The A1C result is reported as a percentage. Here’s how the American Diabetes Association breaks it down:
- 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 prediabetes result is worth paying attention to. It means your body is already struggling to manage blood sugar efficiently, and without changes to diet, activity, or weight, many people in this range progress to type 2 diabetes. The good news is that lifestyle changes at this stage can slow or reverse that trajectory.
Translating A1C to Daily Blood Sugar
If you also check blood sugar with a finger-stick meter, it helps to know what your A1C translates to in everyday numbers. The conversion uses a straightforward formula developed by the ADA: multiply your A1C by 28.7, then subtract 46.7. The result is your estimated average glucose in mg/dL.
Some practical examples:
- A1C of 6.0%: Average blood sugar around 126 mg/dL
- A1C of 7.0%: Average blood sugar around 154 mg/dL
- A1C of 8.0%: Average blood sugar around 183 mg/dL
These averages smooth out the daily highs and lows. Your blood sugar after a meal might spike to 180 mg/dL and drop to 90 mg/dL before the next one. The A1C captures the overall trend, not the peaks and valleys.
A1C Targets for People With Diabetes
For most adults already diagnosed with diabetes, the American Diabetes Association recommends keeping A1C below 7%. That target balances long-term protection against complications (nerve damage, kidney disease, vision problems) with a realistic, sustainable level of blood sugar control.
That said, the target isn’t identical for everyone. Older adults or people managing multiple health conditions may have a slightly higher target, because pushing blood sugar too low carries its own risks, including dangerous episodes of hypoglycemia. Your target depends on your age, overall health, and how long you’ve had diabetes. Testing typically happens at least twice a year for people with stable blood sugar, and more frequently if treatment has recently changed.
How Often You Should Be Tested
Because the test reflects a two-to-three-month window, repeating it more often than every three months doesn’t add useful information. The full effects of a diet change, new exercise routine, or medication adjustment won’t show up until the older red blood cells have been replaced by new ones carrying the updated glucose signature. Testing too soon can make it look like a change isn’t working when it simply hasn’t had enough time.
For people whose blood sugar is well controlled and stable, testing every six months is often sufficient. If your treatment plan has recently changed, a follow-up at three months gives the clearest read on whether the adjustment is working.
When the Test Can Be Inaccurate
The A1C is reliable for most people, but certain conditions can throw off the results. Anything that changes how long your red blood cells survive will skew the number. If red blood cells are destroyed faster than normal, as happens with hemolytic anemia or significant blood loss, the A1C will read falsely low because the cells haven’t been around long enough to accumulate glucose.
Iron deficiency anemia pushes the result in the opposite direction. When iron is low, a chemical byproduct in the blood enhances glucose attachment to hemoglobin, making the A1C read falsely high, even if actual blood sugar is normal. This is particularly relevant during late pregnancy, when iron deficiency is common and can inflate A1C readings in people who don’t have diabetes.
Certain inherited hemoglobin variants, such as sickle cell trait, can also interfere with how the test is measured. People with sickle cell disease face multiple compounding factors: anemia, faster red blood cell turnover, and transfusions all make the A1C unreliable as a marker of blood sugar control. In these cases, alternative tests like fructosamine or glycated albumin provide a more accurate picture. Kidney disease, especially for those on dialysis, similarly makes the A1C less trustworthy.
If you have any of these conditions and your A1C results don’t match how your daily blood sugar readings look, the discrepancy is likely the test, not you.
No Fasting Required
One practical advantage of the A1C: you don’t need to fast before the blood draw. Because the test measures glucose that has accumulated on red blood cells over months, what you ate for breakfast that morning has no meaningful impact on the result. Your doctor may, however, order the A1C alongside other tests like a cholesterol panel that do require fasting, so it’s worth confirming instructions before your appointment.