Hemoglobin A1C is a blood test that measures your average blood sugar level over the past two to three months. Unlike a standard blood sugar check, which captures a single moment in time, A1C gives you and your doctor a longer view of how well your body is managing glucose. The result is expressed as a percentage: below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
How Glucose Attaches to Your Red Blood Cells
The “hemoglobin” in hemoglobin A1C refers to the protein inside red blood cells that carries oxygen throughout your body. When glucose circulates in your bloodstream, some of it naturally sticks to this protein. The process happens in stages: sugar molecules first loosely attach to the surface of hemoglobin, then form a tighter chemical bond that eventually becomes permanent. The more glucose in your blood over time, the more hemoglobin gets coated.
Your red blood cells live for roughly three months before your body replaces them with new ones. That turnover is what gives the A1C test its built-in time window. A blood sample captures red blood cells at different stages of their lifespan, some brand new and some weeks old, providing a weighted average of your blood sugar during that period. More recent weeks influence the result slightly more than earlier ones, since younger red blood cells have had less time to accumulate glucose.
What the Numbers Mean
The American Diabetes Association sets three diagnostic ranges based on A1C results:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
A diagnosis of diabetes typically requires a second confirmatory test, either a repeat A1C or a different type of blood sugar test. Prediabetes is not a diagnosis of disease but a warning that your blood sugar is trending upward and your risk of developing type 2 diabetes is elevated.
If you want to translate your A1C percentage into an everyday blood sugar number, there’s a straightforward formula: multiply your A1C by 28.7, then subtract 46.7. The result is your estimated average glucose in mg/dL. For example, an A1C of 7% translates to an estimated average blood sugar of about 154 mg/dL. An A1C of 6% works out to roughly 126 mg/dL. This conversion helps bridge the gap between the percentage your doctor reports and the kind of number you’d see on a home glucose meter.
Why A1C Matters for Long-Term Health
The reason doctors track A1C rather than relying solely on daily glucose readings is that sustained high blood sugar causes damage over months and years. Consistently elevated levels harm small blood vessels in the eyes, kidneys, and nerves, and increase the risk of heart attack and stroke. Research shows a clear dose-response relationship: the higher your A1C climbs above the normal range, the greater the risk of these complications.
The relationship is not perfectly linear, though. Studies in people with kidney disease, for instance, show that complication risk rises significantly once A1C reaches about 7.2%, with the sharpest increases above 9.5%. Interestingly, very low A1C levels (below about 5.4%) can also be associated with higher risk in certain populations, particularly among people taking diabetes medications that can cause dangerously low blood sugar. This is one reason doctors set individualized targets rather than simply aiming for the lowest possible number.
How Often You’ll Be Tested
If your blood sugar is well controlled and your treatment hasn’t changed recently, you can expect an A1C test about twice a year. If you’ve recently started a new medication, adjusted your dose, or aren’t hitting your target, your doctor will likely check it every three months. That quarterly schedule aligns perfectly with the red blood cell lifespan, giving just enough time for the previous result to fully cycle out.
One practical advantage of the A1C test: you don’t need to fast. Blood can be drawn at any time of day regardless of when you last ate, which makes it easier to schedule than fasting glucose tests. The draw itself is a standard blood sample from your arm.
When A1C Results Can Be Misleading
Because A1C depends on hemoglobin inside red blood cells, anything that changes your red blood cells can skew the result. Conditions that shorten the lifespan of red blood cells, such as sickle cell disease or other hemoglobin variants, tend to produce falsely low A1C readings. The cells don’t live long enough to accumulate the expected amount of glucose. Certain types of anemia can have a similar effect.
On the other hand, conditions that slow red blood cell turnover, like iron deficiency anemia, can push A1C results artificially high. The cells stick around longer and collect more glucose than they normally would. Pregnancy, recent blood loss or transfusion, and chronic kidney disease can also interfere with accuracy.
If your doctor suspects your A1C isn’t reflecting your true blood sugar levels, they may use alternative tests like fructosamine (which measures glucose attachment to other blood proteins over a shorter window) or rely more heavily on direct glucose monitoring.
A1C Targets Are Personal
For most adults with diabetes, the standard target is below 7%. But your ideal number depends on your age, how long you’ve had diabetes, what medications you take, and whether you’re prone to episodes of low blood sugar. Older adults or people with other serious health conditions often have a more relaxed target of 7.5% or even 8%, because the risk of driving blood sugar too low with aggressive treatment can outweigh the benefits of a tighter number.
For people with prediabetes, the goal is straightforward: keep A1C from crossing 6.5%. Weight loss of 5% to 7% of body weight, regular physical activity, and dietary changes can often bring A1C back into the normal range or at least slow its progression. Even small reductions matter. Dropping your A1C by half a percentage point meaningfully lowers your risk of the vascular damage that high blood sugar causes over time.