A1C hemoglobin (also written as HbA1c) is a blood test that measures your average blood sugar level over the past two to three months. Unlike a standard blood sugar check that captures a single moment, the A1C gives a broader picture of how well your body has been managing glucose over time. It’s the primary tool used to diagnose prediabetes and diabetes, and for people already living with diabetes, it’s the standard for tracking how well treatment is working.
How Glucose Attaches to Hemoglobin
Hemoglobin is the protein inside red blood cells that carries oxygen throughout your body. When glucose circulates in your bloodstream, some of it naturally sticks to hemoglobin through a chemical reaction. The glucose bonds permanently to the hemoglobin molecule, creating what’s called glycated hemoglobin. Once that bond forms, it doesn’t come undone. The sugar stays attached for the entire life of that red blood cell.
Red blood cells live about 90 to 120 days before your body replaces them. That’s why the A1C reflects a two- to three-month window: the test measures the percentage of hemoglobin molecules that have glucose attached. Higher blood sugar over that period means more hemoglobin gets coated with glucose, which means a higher A1C reading. Lower blood sugar means less coating and a lower number.
What the Numbers Mean
The A1C result is reported as a percentage. The CDC uses these thresholds for diagnosis:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
Within the prediabetes range, higher numbers carry greater risk. Someone at 6.3% is closer to developing diabetes than someone at 5.8%, even though both fall in the same category.
You can also translate an A1C percentage into an estimated average glucose (eAG) using a simple formula: multiply the A1C by 28.7, then subtract 46.7. The result is your approximate average blood sugar in mg/dL. For example, an A1C of 7% works out to an eAG of about 154 mg/dL. This conversion can make the number more intuitive if you’re used to checking blood sugar with a home meter.
Why It Doesn’t Require Fasting
One practical advantage of the A1C test is that you don’t need to fast beforehand. A fasting glucose test requires you to skip food for at least eight hours, and the result only tells you what your blood sugar was at that exact moment. The A1C captures months of data from a single blood draw, so eating breakfast before your appointment won’t affect the result. Your doctor can order it during a routine visit without any special preparation.
How Often You Should Get Tested
If you have diabetes and your treatment has recently changed, or you’re struggling to meet your blood sugar goals, testing every three months is typical. If your numbers are stable and you’re meeting your targets, every six months is generally enough. For people without diabetes, the test is part of routine screening, especially after age 45 or if you have risk factors like obesity, family history, or a sedentary lifestyle.
A1C Targets Vary by Person
The American Diabetes Association recommends an A1C of 7% or lower for most adults with diabetes. Some guidelines suggest 6.5% or lower. But these aren’t one-size-fits-all numbers.
For older adults, targets are often more flexible. A healthy, active person in their 70s might aim for 7% or below, just like a younger adult. But for someone who is frail, has other serious health conditions, or has a limited life expectancy, doctors often set a higher target. The reason is practical: aggressively lowering blood sugar increases the risk of hypoglycemia (dangerously low blood sugar episodes), which can cause falls, confusion, and hospitalization. For older people, those immediate risks can outweigh the long-term benefits of tighter control. The goal shifts toward avoiding symptoms and maintaining quality of life rather than hitting a specific number.
Younger adults and people early in their diabetes diagnosis generally benefit more from keeping A1C low, since years of elevated blood sugar increase the risk of damage to the eyes, kidneys, and nerves.
When the A1C Can Be Misleading
The test assumes your red blood cells live a normal lifespan. When that assumption breaks down, the A1C may not reflect your true blood sugar average.
Conditions that shorten red blood cell lifespan, like hemolytic anemia or significant blood loss, cause red blood cells to turn over faster. Newer cells have had less time to accumulate glucose, so the A1C reads falsely low. You could have high blood sugar and still get a reassuring-looking number.
Iron deficiency anemia pushes in the opposite direction. It tends to increase A1C readings, making blood sugar look worse than it actually is. This is particularly relevant during late pregnancy, when iron deficiency is common. A non-diabetic pregnant woman can get an A1C result that looks elevated purely because of iron status, not because of a blood sugar problem.
Genetic hemoglobin variants also affect accuracy. People with sickle cell trait, hemoglobin C trait, or other hemoglobin variants may get unreliable results depending on the lab method used. Kidney failure creates another layer of complexity: chemically modified hemoglobin in people on dialysis can interfere with the test, and A1C often underestimates blood sugar in this group. For these patients, doctors sometimes use alternative markers like glycated albumin instead.
If you have any of these conditions, your doctor should interpret your A1C with that context in mind, or use a different test altogether.
A1C vs. Daily Blood Sugar Checks
The A1C and a glucose meter tell you different things. A finger-stick glucose reading shows exactly where your blood sugar is right now. It’s useful for making immediate decisions: adjusting insulin, deciding whether to eat, figuring out why you feel off. But it only captures a snapshot.
The A1C smooths out the daily spikes and dips into a single average. Two people can have the same A1C of 7% with very different daily patterns. One might have stable blood sugar hovering around 154 mg/dL. The other might swing between 60 and 250 throughout the day. The A1C won’t distinguish between these two scenarios, which is why many doctors look at both the A1C and daily glucose data (or continuous glucose monitor readings) to get the full picture.
Think of daily glucose checks as the weather forecast and the A1C as the climate. Both are useful, and neither replaces the other.