Hemoglobin A1c (often just called A1c) is a blood test that measures your average blood sugar level over the past three months. It’s reported as a percentage: below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes. Unlike a finger-stick glucose reading that captures a single moment, A1c gives a much wider picture of how your body has been handling sugar over time.
How the Test Works
Hemoglobin is a protein inside your red blood cells that carries oxygen through your bloodstream. When glucose circulates in your blood, some of it naturally attaches to hemoglobin in a process called glycation. Everyone has some sugar-coated hemoglobin, but the more glucose in your blood, the more hemoglobin gets coated, and the higher your A1c percentage climbs.
The reason the test reflects roughly three months is simple biology: red blood cells live about 120 days before your body replaces them. So the hemoglobin in your blood at any given time carries a chemical record of your blood sugar levels over that entire lifespan. A blood draw captures red blood cells of all different ages, giving a weighted average that leans slightly toward the most recent four to six weeks.
What the Numbers Mean
The American Diabetes Association uses three ranges for diagnosis:
- Below 5.7%: Normal blood sugar control
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is elevated but not yet in the diabetes range
- 6.5% or higher: Diabetes
For people already managing diabetes, most guidelines recommend aiming for an A1c below 7%, though your target may be higher or lower depending on your age, other health conditions, and how long you’ve had diabetes.
Translating A1c to Daily Blood Sugar
A percentage can feel abstract if you’re used to seeing blood sugar as a number on a glucose meter. There’s a formula that converts A1c into an estimated average glucose (eAG), which is the kind of number you’d see on a home meter. The conversion multiplies your A1c by 28.7 and subtracts 46.7 to get a value 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
- A1c of 9.0%: roughly 212 mg/dL average
These are averages, so they smooth out the daily highs and lows. Someone with an A1c of 7% might regularly swing between 80 and 250 mg/dL throughout the day. The A1c alone doesn’t reveal those swings, which is why some people also use continuous glucose monitors or daily finger sticks alongside periodic A1c tests.
No Fasting Required
One practical advantage of the A1c test is that you don’t need to fast beforehand. Blood can be drawn at any time of day regardless of when you last ate. That’s because the test measures glucose that has accumulated on hemoglobin over months, not the glucose currently floating in your bloodstream. A meal an hour before the draw won’t change your result. Neither will a single stressful day or a tough workout. Large shifts in blood sugar over the past month will show up, but brief spikes and dips won’t register.
How Often to Get Tested
Testing frequency depends on where you fall on the blood sugar spectrum. If your results come back normal, the American Diabetes Association recommends retesting at least every three years, or sooner if your risk profile changes (for example, significant weight gain or new symptoms like increased thirst and frequent urination). People with prediabetes should test yearly. If you’ve been diagnosed with diabetes and your blood sugar is well controlled, testing twice a year is typical. When treatment changes or blood sugar is not at target, testing every three months helps track whether adjustments are working.
When A1c Results Can Be Misleading
The test assumes your red blood cells live a normal lifespan and turn over at a normal rate. Anything that disrupts that assumption can push results artificially high or low.
Conditions That Falsely Raise A1c
When red blood cells stick around longer than usual, they accumulate more glucose coating, inflating the number. Iron deficiency anemia is the most common culprit. Deficiencies in vitamin B12 or folate have the same effect. People who have had their spleen removed (which normally filters out old red blood cells) also tend to get falsely elevated readings. Chronic heavy alcohol use can raise results too, because alcohol produces a compound that mimics glycated hemoglobin in some lab tests. Chronic use of aspirin-type medications or opioids has also been linked to falsely high readings, though the exact mechanisms aren’t fully understood.
Conditions That Falsely Lower A1c
The opposite happens when red blood cells are destroyed or lost faster than normal. Hemolytic anemia, chronic blood loss, and an enlarged spleen all shorten red blood cell lifespan, giving glucose less time to attach. End-stage kidney disease typically produces falsely low A1c values because the associated chronic anemia speeds up red blood cell turnover. Pregnancy also shortens red blood cell lifespan from the usual 120 days to closer to 90 days, so A1c readings in pregnant individuals may underestimate true blood sugar levels.
If any of these conditions apply to you, your provider may use alternative tests like fructosamine (which measures sugar-coated proteins over a two- to three-week window) or rely more heavily on daily glucose monitoring to get an accurate picture. The A1c is a powerful screening and management tool, but it works best when your red blood cells behave predictably.
Why A1c Matters Beyond Diagnosis
A1c isn’t just a one-time diagnostic test. For people living with diabetes, it’s the primary scorecard for long-term blood sugar management. Large studies have consistently shown that each percentage-point drop in A1c reduces the risk of complications affecting the eyes, kidneys, and nerves. Going from an A1c of 9% down to 7%, for instance, represents a meaningful reduction in the likelihood of damage to small blood vessels over time.
That’s what makes the test valuable for tracking progress. Diet changes, exercise habits, and medications all take weeks to show their full effect on blood sugar. A single glucose reading can’t capture that trajectory, but an A1c taken every three to six months reveals whether the overall trend is moving in the right direction.