An A1C test shows your average blood sugar level over the past two to three months, expressed as a percentage. Unlike a finger stick or fasting glucose test that captures a single moment, A1C reveals the bigger picture of how your body has been handling sugar over time. It’s used to screen for prediabetes and diabetes, and to track how well blood sugar management is working for people already diagnosed.
How A1C Measures Blood Sugar Over Time
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. This process, called glycation, happens continuously and at a rate proportional to how much glucose is circulating. The more sugar in your blood, the more hemoglobin gets coated with it. An A1C test measures the percentage of your hemoglobin that has glucose attached.
The reason A1C reflects roughly two to three months of blood sugar history comes down to the lifespan of red blood cells. These cells have a half-life of about 50 days and are constantly being recycled and replaced. At any given time, your blood contains a mix of newer and older red blood cells, each carrying a record of the glucose they’ve been exposed to since they were made. The test captures a weighted average of that entire span, with more recent weeks influencing the number more than earlier ones.
What the Numbers Mean
The American Diabetes Association defines three ranges:
- Below 5.7%: Normal blood sugar levels
- 5.7% to 6.4%: Prediabetes range
- 6.5% or higher: Diabetes range
These percentages can feel abstract, so it helps to convert them into estimated average glucose, the kind of number you’d see on a home glucose meter. An A1C of 6% corresponds to an average blood sugar of about 126 mg/dL. At 7%, that average climbs to roughly 154 mg/dL. An A1C of 8% means your blood sugar has been averaging around 183 mg/dL, and at 9% it’s approximately 212 mg/dL. Each one-point increase in A1C translates to an average glucose increase of about 28 to 30 mg/dL.
For people managing diabetes, the typical target is an A1C below 7%, though your specific goal may be higher or lower depending on your age, health, and how long you’ve had diabetes.
What A1C Doesn’t Show
The biggest limitation of A1C is that it’s an average, and averages hide a lot. Two people can have the same A1C of 7% while experiencing very different daily blood sugar patterns. One person might hold steady between 130 and 170 mg/dL most of the day. Another might swing from dangerous lows of 50 mg/dL overnight to spikes above 250 mg/dL after meals, with the peaks and valleys canceling each other out to produce the same average.
A1C is particularly insensitive to hypoglycemia, those episodes where blood sugar drops low enough to cause shakiness, confusion, or fainting. These episodes can be clinically dangerous even when they’re brief, and they leave almost no trace in an A1C reading. It also can’t tell you when your blood sugar tends to rise, whether that’s after breakfast, during sleep, or in response to stress.
This is why continuous glucose monitors have become an important complement to A1C testing. These devices track blood sugar every few minutes and generate a metric called time in range: the percentage of the day your glucose stays between 70 and 180 mg/dL. A time in range of 70% roughly corresponds to an A1C of 7%, but it gives you and your doctor far more actionable detail. A 10% improvement in time in range correlates with about a 0.6 to 0.8% drop in A1C, and more time in range is linked to lower risk of diabetes-related complications affecting the eyes, kidneys, nerves, and heart.
Factors That Can Skew Your Results
Several conditions can make an A1C reading misleadingly high or low, even when your actual blood sugar control hasn’t changed.
Iron deficiency anemia is one of the most common culprits. It tends to push A1C readings artificially higher, and treating the iron deficiency with supplements lowers A1C even in people without diabetes. This is especially relevant during late pregnancy, when iron deficiency is common and can inflate A1C results in women who otherwise have normal blood sugar.
On the other hand, any condition that shortens the lifespan of red blood cells will pull A1C readings falsely low. This includes hemolytic anemias, recent significant blood loss, and recovery from acute bleeding. When red blood cells are destroyed or replaced faster than usual, they simply haven’t been around long enough to accumulate as much glucose on their hemoglobin.
Genetic hemoglobin variants also interfere with the test. People with sickle cell trait, hemoglobin C trait, or other variants can get unreliable results depending on the specific laboratory method used. For people with sickle cell disease, the combination of anemia, rapid red blood cell turnover, and transfusion needs makes A1C especially unreliable as a marker of blood sugar control. In these cases, alternative tests like fructosamine (which measures sugar attached to blood proteins over a shorter window) or direct glucose monitoring become necessary.
Kidney failure also distorts the test. Dialysis patients tend to get A1C readings that underestimate their actual blood sugar levels, which can give a false sense of good control.
Differences Across Racial and Ethnic Groups
A1C cutoffs were established largely from studies of White populations, and growing evidence suggests they don’t perform equally well across all groups. Research compiled by the CDC found that African Americans tend to have higher A1C values across the entire blood sugar spectrum compared to White individuals, even at the same actual glucose levels. This means the standard cutoffs may overestimate the severity of blood sugar problems in African Americans.
The pattern differs for other populations of African descent. In studies of Afro-Caribbean and African-born individuals, A1C actually underestimated blood sugar levels, leading to more missed diagnoses of both prediabetes and diabetes when compared to oral glucose tolerance tests. These disparities are driven partly by differences in red blood cell lifespan and rates of hemoglobin variants, conditions that disproportionately affect people of African descent. It’s a meaningful gap: in one study of Afro-Caribbean people, using the standard A1C cutoff of 6.5% missed a significant number of diabetes cases that an oral glucose test would have caught.
How Often to Get Tested
If you have diabetes and your A1C is within your goal range, testing every six months is generally sufficient. If your treatment plan has recently changed, or your numbers aren’t where they need to be, your doctor will likely check it more frequently, often at every visit. For people without diabetes who are being screened, A1C is typically part of routine blood work during annual checkups, especially after age 45 or earlier if you have risk factors like obesity, a family history of diabetes, or a history of gestational diabetes.
No special preparation is needed for the test. Unlike a fasting glucose test, you don’t need to skip meals beforehand. A1C can be drawn from a standard blood sample at any time of day, which is one reason it became such a widely used screening tool.