A1c (also called HbA1c or hemoglobin A1c) is a blood test that measures your average blood sugar over the past two to three months. It’s expressed as a percentage: a normal result is below 5.7%, while 6.5% or higher indicates diabetes. Unlike a finger-stick glucose reading that captures a single moment, A1c gives a longer view of how well your body is managing blood sugar over time.
How A1c Works in Your Body
Glucose circulating in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. The more sugar in your blood, the more hemoglobin gets coated. This coating is permanent for the life of each red blood cell, which is about three months on average. So when a lab measures the percentage of your hemoglobin that’s sugar-coated, the result reflects your blood sugar levels across that entire lifespan.
This is why A1c is sometimes called a “three-month average.” It isn’t a snapshot like a fasting glucose test. It captures the cumulative effect of daily blood sugar highs and lows, meals, exercise, stress, and sleep over roughly 8 to 12 weeks. The most recent weeks tend to contribute more heavily to the final number, since newer red blood cells have had less time to accumulate glucose.
What the Numbers Mean
The CDC uses these ranges to classify A1c results:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
A result in the prediabetes range means your blood sugar is higher than normal but hasn’t crossed the diabetes threshold. About one in three American adults fall into this category, and many don’t know it. The good news is that prediabetes is often reversible with changes to diet, physical activity, and weight.
If your result is 6.5% or higher on two separate tests, that typically confirms a diabetes diagnosis. From there, A1c becomes a tool for monitoring how well your blood sugar management plan is working over time.
Translating A1c to Daily Blood Sugar
An A1c percentage can feel abstract. To make it more concrete, labs often convert it into an estimated average glucose (eAG), which is the kind of number you’d see on a glucose meter. The conversion follows a simple formula: multiply your A1c by 28.7, then subtract 46.7. The result is your estimated average in mg/dL.
Here’s what that looks like in practice:
- A1c of 5.7%: roughly 117 mg/dL average
- A1c of 6.5%: roughly 140 mg/dL average
- A1c of 7.0%: roughly 154 mg/dL average
- A1c of 8.0%: roughly 183 mg/dL average
Keep in mind that “average” means your actual blood sugar swings above and below this number throughout each day. Two people with the same A1c can have very different daily patterns. One might have steady, moderate readings, while the other swings between highs and lows that happen to average out to the same number.
Why Small Changes in A1c Matter
Lowering your A1c, even modestly, has a measurable effect on long-term health. In a large prospective study of people with type 2 diabetes, each 1% drop in A1c was associated with a 37% reduction in risk for microvascular complications (damage to the small blood vessels in the eyes, kidneys, and nerves) and a 14% reduction in risk for heart attack. That means going from 8% to 7% isn’t just a number on a lab report. It translates directly into lower odds of the complications that make diabetes dangerous.
This is why A1c is the primary metric doctors use to track diabetes management over time. A single high fasting glucose reading could reflect a bad night’s sleep or a stressful morning. A high A1c reflects a pattern that needs attention.
Target Goals Aren’t the Same for Everyone
For most adults with diabetes, the general target is an A1c below 7%. But that number gets adjusted based on your circumstances. Pregnant women with preexisting diabetes are typically advised to aim below 6% during pregnancy, since even slightly elevated blood sugar affects fetal development. Before conception, the recommended target is below 6.5%.
On the other end of the spectrum, people who experience frequent episodes of dangerously low blood sugar (hypoglycemia) may be given a less aggressive target, such as below 7% or even higher, to avoid the risks that come with pushing blood sugar too low. Older adults with multiple health conditions often have relaxed targets as well, since the benefits of tight control take years to materialize and the risks of low blood sugar increase with age.
How Often You Should Get Tested
Testing frequency depends on how stable your blood sugar is. If you’re actively adjusting your treatment plan or haven’t yet reached your target, testing every three months makes sense. It aligns perfectly with the red blood cell lifespan that A1c measures, giving you a fresh and complete picture each time.
Once you’ve achieved stable control, testing every six months is generally sufficient. Some situations call for more frequent checks: children and adolescents with type 1 diabetes, people planning a pregnancy, or anyone who has recently made significant changes to their medications or lifestyle.
When A1c Results Can Be Misleading
A1c is reliable for most people, but certain conditions can skew the results. Since the test depends on hemoglobin and red blood cell lifespan, anything that changes either of those will affect accuracy.
Iron deficiency anemia can push A1c readings falsely high. When iron is low, red blood cells live longer than usual, giving hemoglobin more time to accumulate glucose. Studies have shown that treating the iron deficiency with supplements lowers A1c even when actual blood sugar hasn’t changed. This is especially relevant during late pregnancy, when iron deficiency is common and can inflate A1c in women who aren’t diabetic.
Conditions that shorten red blood cell lifespan have the opposite effect. Hemolytic anemia, significant blood loss, or receiving a blood transfusion can all produce falsely low A1c readings, because the red blood cells being measured haven’t been around long enough to accumulate a representative amount of glucose.
Certain inherited hemoglobin variants, such as sickle cell trait and hemoglobin C trait, can also interfere with A1c accuracy. The effect depends on the specific variant and the laboratory method used to run the test. People with sickle cell disease face additional complications: the combination of anemia, rapid red cell turnover, and transfusions makes A1c an unreliable marker of blood sugar control. In these cases, alternative tests that measure glucose attachment to other proteins (like albumin in the blood) give a more accurate picture.
Chronic kidney disease, which is common in people with long-standing diabetes, can also distort A1c results. For patients on dialysis, A1c tends to underestimate actual blood sugar levels, meaning the real situation may be worse than the number suggests.
If you have any of these conditions, your lab results should be interpreted with that context in mind. The test itself is straightforward (a simple blood draw, no fasting required), but the number it produces tells the fullest story when your red blood cells are behaving normally.