A1C is a blood test that measures your average blood sugar over the past two to three months. Unlike a finger stick that captures your glucose at a single moment, A1C reveals the bigger picture of how your body has been handling sugar over time. It’s one of the primary tools used to diagnose prediabetes and diabetes, and to monitor how well blood sugar is being managed.
How the A1C Test Works
Hemoglobin is a protein inside your red blood cells that carries oxygen. As blood circulates, glucose in your bloodstream naturally sticks to hemoglobin. The more sugar in your blood, the more hemoglobin gets coated. The A1C test measures the percentage of hemoglobin that has glucose attached to it.
Red blood cells live for about three months before your body replaces them. That turnover is what gives the test its time window. An A1C result reflects the glucose buildup on hemoglobin across that entire lifespan, which is why it serves as a rolling average rather than a snapshot. A single high-sugar meal or a stressful week won’t dramatically change your number, but consistently elevated blood sugar will.
The test doesn’t require fasting. You can eat and drink normally beforehand, which makes it more convenient than a fasting glucose test.
What the Numbers Mean
A1C results are reported as a percentage. The National Institute of Diabetes and Digestive and Kidney Diseases uses these thresholds:
- 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 into diabetes. This is often a window where lifestyle changes (diet, exercise, weight loss) can slow or prevent progression. A result at 6.5% or higher, confirmed with a repeat test, is used to diagnose type 2 diabetes.
For people already living with diabetes, targets are individualized. The American Diabetes Association’s 2025 guidelines recommend that clinicians set glycemic goals based on a person’s overall health, risk of low blood sugar episodes, and personal preferences. There isn’t one universal magic number. Someone who is young and otherwise healthy may aim for a tighter target, while someone older or managing multiple conditions may have a more relaxed goal.
Converting A1C to Average Blood Sugar
If you’re used to seeing blood sugar in mg/dL from a glucose meter or continuous monitor, you can translate your A1C into a more familiar number. Researchers developed a formula for this: multiply your A1C by 28.7, then subtract 46.7. The result is your estimated average glucose (eAG) in mg/dL.
For example, an A1C of 7% translates to an estimated average glucose of about 154 mg/dL. An A1C of 6% works out to roughly 126 mg/dL. Many lab reports now include this conversion automatically, which makes it easier to connect your A1C to the daily numbers you see on a meter.
Why A1C and Daily Readings Sometimes Disagree
It’s common for people to notice their A1C doesn’t perfectly match the average on their glucose meter or continuous monitor. There are several reasons for this.
First, daily readings tend to be taken at specific times, often before meals or at bedtime. Those snapshots may miss post-meal spikes or overnight lows, so your meter average could look different from the true 24-hour average the A1C reflects. Second, individual biology plays a role. Some people glycate hemoglobin faster than others, meaning glucose sticks to their hemoglobin more readily. Researchers call this the “glycation gap,” and it means two people with identical average blood sugar can have slightly different A1C results.
Racial and ethnic background can also influence A1C levels independent of blood sugar, a pattern supported by multiple studies. This doesn’t mean the test is useless for any group, but it’s one reason clinicians sometimes pair A1C with other measures for a fuller picture.
Conditions That Can Skew Results
Because the test depends on hemoglobin and the normal lifespan of red blood cells, anything that alters either one can throw off the result.
Iron deficiency anemia tends to push A1C readings falsely high. That’s because iron-deficient red blood cells live longer and accumulate more glucose. On the other hand, conditions that shorten red blood cell lifespan, like sickle cell disease or significant blood loss, can make A1C appear falsely low because younger red blood cells haven’t had as much time to collect glucose.
Kidney disease, liver failure, and certain hemoglobin variants can also affect accuracy. High triglycerides and chronic heavy alcohol use have been linked to falsely elevated results with some testing methods. If you have any of these conditions, your doctor may rely on alternative ways to track blood sugar, such as a fructosamine test that measures a shorter window of about two to three weeks.
How Often You Should Get Tested
Testing frequency depends on where you are in managing your blood sugar. If you have diabetes and are still working toward your target, or you’ve recently changed medications or made major lifestyle shifts, testing every three months makes sense. That gives enough time for a new batch of red blood cells to reflect the change.
Once your blood sugar is stable and consistently meeting your goal, testing every six months is generally sufficient. More frequent testing may be recommended for children with type 1 diabetes, people planning pregnancy, or anyone experiencing rapid blood sugar fluctuations.
If you don’t have diabetes, an A1C test during routine bloodwork is a straightforward way to screen for prediabetes, especially if you have risk factors like a family history of diabetes, excess weight, or a sedentary lifestyle. Since no fasting is required, it can be done at any appointment.