Hemoglobin A1c is a blood test that measures your average blood sugar level over the past three months. Unlike a standard glucose test that captures a single moment in time, the A1c gives a longer view of how well your body is managing blood sugar. It’s one of the primary tools used to screen for prediabetes and diabetes, and to monitor how well diabetes treatment is working.
How the Test Works
Hemoglobin is a protein inside red blood cells that carries oxygen through your bloodstream. As blood sugar circulates, glucose molecules attach to hemoglobin in a process called glycation. The higher your blood sugar runs, the more glucose coats your hemoglobin, and the higher your A1c percentage climbs.
Red blood cells live for about three months before your body replaces them. Because of this natural turnover cycle, the A1c test captures a rolling average of your blood sugar over roughly 90 days. A single high-sugar day won’t spike the number, and a single good day won’t bring it down. It reflects the overall pattern.
The result is reported as a percentage. A higher percentage means more of your hemoglobin has been coated with glucose, which means your blood sugar has been running higher on average.
What the Numbers Mean
The CDC uses these thresholds to interpret A1c results:
- Below 5.7%: Normal blood sugar control
- 5.7% to 6.4%: Prediabetes
- 6.5% or above: Diabetes
A result in the prediabetes range means your blood sugar is higher than normal but hasn’t crossed into diabetes territory. This is the window where lifestyle changes, like adjusting your diet and increasing physical activity, can often prevent or delay progression to type 2 diabetes.
A1c Translated to Daily Blood Sugar
If you’ve ever checked your blood sugar with a finger-prick meter, you know those readings come in mg/dL. Your A1c percentage can be converted to an estimated average glucose, which makes it easier to connect the lab result to what you’d see on a home monitor. The conversion is based on a well-validated formula from a study published in Diabetes Care.
- A1c of 5%: Average blood sugar around 97 mg/dL
- A1c of 6%: Around 126 mg/dL
- A1c of 7%: Around 154 mg/dL
- A1c of 8%: Around 183 mg/dL
- A1c of 9%: Around 212 mg/dL
- A1c of 10%: Around 240 mg/dL
- A1c of 12%: Around 298 mg/dL
These are averages, so your actual daily readings will swing above and below these numbers. Two people with the same A1c can have very different daily patterns. One might have steady blood sugar while another swings between highs and lows that average out to the same number.
No Fasting Required
One practical advantage of the A1c test is that you don’t need to fast before your blood draw. Because it measures glucose buildup over months rather than your blood sugar at that moment, eating breakfast beforehand won’t change the result. This makes it more convenient than a fasting glucose test, which requires you to skip food for at least eight hours.
The test itself is a simple blood draw, and results typically come back within a day or two.
How Often You’ll Be Tested
If you have diabetes and your blood sugar is well-controlled, you’ll typically get an A1c test at least twice a year. If your numbers aren’t at target or your treatment plan has recently changed, your doctor may test more frequently, sometimes every three months. For people without diabetes, the test is used as a screening tool during routine checkups, especially if you have risk factors like being overweight, having a family history of diabetes, or being over 45.
When Results Can Be Misleading
The A1c test is reliable for most people, but certain conditions can throw the results off. Because the test depends on how long red blood cells survive and how hemoglobin behaves, anything that disrupts those factors can skew the number in either direction.
Conditions That Push A1c Falsely High
Iron deficiency anemia is one of the most common causes of a falsely elevated A1c. When your body is low on iron, red blood cells survive longer than usual, giving glucose more time to accumulate on hemoglobin. Vitamin B-12 and folate deficiency anemias have the same effect. People who have had their spleen removed also tend to get artificially high readings because their red blood cells circulate longer without the spleen filtering them out. Chronic alcohol consumption can also push A1c up through the formation of a compound that mimics glycated hemoglobin in some lab tests.
Conditions That Push A1c Falsely Low
On the other end, anything that shortens the lifespan of red blood cells will make A1c look lower than it should. Acute or chronic blood loss, certain types of anemia where red blood cells break down prematurely, and an enlarged spleen can all produce falsely reassuring results. People with end-stage kidney disease often have low A1c values because their chronic anemia causes rapid red blood cell turnover. This is especially important to recognize because kidney disease and diabetes frequently go hand in hand.
Hemoglobin Variants
Inherited hemoglobin variants, most commonly hemoglobin S (associated with sickle cell disease) and hemoglobin C, can interfere with A1c accuracy. People who carry one copy of these variants can still get reliable A1c results if the right lab method is used. But for those with two copies (like sickle cell disease), the A1c test is generally not reliable, and alternative methods of tracking blood sugar are necessary.
A1c During Pregnancy
The A1c test has significant limitations during pregnancy. In the first trimester, it reflects blood sugar fairly accurately. But as pregnancy progresses, it becomes increasingly unreliable. Red blood cells have a shorter lifespan during pregnancy (closer to 90 days instead of the usual 120), and the body produces more of them, both of which drag the A1c reading down. By the late second trimester, A1c can underestimate how high blood sugar actually is, potentially giving false reassurance at exactly the time when gestational diabetes typically develops.
For this reason, gestational diabetes is diagnosed using an oral glucose tolerance test rather than A1c. Ongoing monitoring during pregnancy relies on regular finger-prick blood sugar checks rather than periodic A1c measurements.