Hemoglobin A1c measures the percentage of your red blood cells’ hemoglobin that has glucose permanently attached to it. Because red blood cells live for roughly 90 to 120 days, the test captures a rolling average of your blood sugar over the past two to three months, rather than a single moment in time. That makes it far more useful than a finger-stick glucose reading for understanding long-term blood sugar control.
How Glucose Attaches to Hemoglobin
Hemoglobin is the protein inside red blood cells that carries oxygen. When glucose circulates in your bloodstream, some of it sticks to hemoglobin through a process called glycation. This isn’t something your body does on purpose; it happens spontaneously whenever glucose and hemoglobin come into contact. The higher your blood sugar, the more glucose attaches.
The bond starts out loosely, then rearranges into a stable, permanent connection. Once glucose locks onto hemoglobin this way, it stays there for the entire life of that red blood cell. So when a lab measures the percentage of hemoglobin carrying glucose, it’s effectively reading a biological record of every blood sugar high and low you’ve experienced over the previous few months. Recent weeks are weighted a bit more heavily, since newer red blood cells make up a larger share of the total at any given time.
What the Numbers Mean
The A1c result is reported as a percentage. According to the American Diabetes Association’s 2025 standards:
- Below 5.7%: normal
- 5.7% to 6.4%: prediabetes
- 6.5% or higher: diabetes
These thresholds are used for diagnosis, but A1c is equally important for people already living with diabetes. The ADA suggests a target of 7% for most nonpregnant adults with diabetes, which corresponds to an estimated average glucose of about 154 mg/dL. Your doctor may set a different personal target depending on your age, how long you’ve had diabetes, and other health factors.
A1c vs. Daily Glucose Readings
A finger-stick or continuous glucose monitor tells you what your blood sugar is right now. That snapshot is useful for making immediate decisions about food, exercise, or medication, but it can’t tell you how well things have been going overall. You could check your glucose on a calm morning after sleeping well and eating carefully, and the number might look fine even if your blood sugar has been running high the rest of the time.
A1c fills that gap. It smooths out all the daily fluctuations into one number that reflects the bigger picture. The tradeoff is that it can’t reveal patterns, like whether your blood sugar spikes after lunch or drops overnight. For the most complete view, many people use both: daily monitoring for real-time adjustments and A1c testing every few months to confirm those adjustments are working.
Why Small Changes in A1c Matter
The difference between, say, 8% and 7% might not sound dramatic, but the health implications are significant. Data from the landmark United Kingdom Prospective Diabetes Study found that a person averaging 7% had a 37% lower risk of microvascular complications compared to someone averaging 8%. Microvascular complications are the ones that damage small blood vessels: vision loss from retinopathy, kidney disease, and nerve damage in the hands and feet. That same one-percentage-point difference was linked to a 21% lower risk of any diabetes-related health problem.
These numbers illustrate why clinicians treat A1c as the single most important marker of diabetes management. Even modest improvements translate into meaningfully lower odds of long-term damage.
How Often to Get Tested
The CDC recommends testing every six months if your blood sugar is well controlled and you’re meeting your treatment goals. If your treatment has recently changed, or you’re having trouble keeping blood sugar in range, every three months is more appropriate. Since the test reflects roughly the past two to three months, testing more frequently than that wouldn’t add much new information.
When the Test Can Be Misleading
A1c depends on two things: your actual blood sugar levels and the normal lifespan of your red blood cells. Anything that disrupts either one can skew the result.
Conditions that shorten how long red blood cells survive, like certain anemias, sickle cell disease (HbSS), hemoglobin C disease (HbCC), significant kidney disease, or liver failure, can produce A1c readings that don’t match your true average glucose. When red blood cells are destroyed or replaced faster than usual, they have less time to accumulate glucose, which can push A1c falsely low. The opposite can happen too: conditions that extend red blood cell life, like iron deficiency anemia, can make A1c appear falsely high because those cells spend more time collecting glucose.
Hemoglobin variants also play a role. There are hundreds of known variants, but the most common ones are hemoglobin S, E, C, and D. Depending on the lab method used, these variants can interfere with the measurement and cause inaccurate readings in either direction. If you carry a hemoglobin variant, your doctor may rely on alternative tests, like fructosamine, which measures glycation of blood proteins over a shorter, two-to-three-week window.
Pregnancy, recent blood transfusions, and chronic blood loss can also affect accuracy. If your A1c results seem inconsistent with your daily glucose readings, one of these factors may be the reason.