A hemoglobin A1c test measures your average blood sugar level over the past two to three months. It does this by checking what percentage of your red blood cells have glucose (sugar) attached to them. The result is reported as a percentage: below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher signals diabetes. No fasting is required before the test.
How the Test Works
Hemoglobin is a protein inside your red blood cells that carries oxygen. When glucose circulates in your bloodstream, some of it naturally sticks to hemoglobin. The higher your blood sugar has been, the more glucose-coated hemoglobin you’ll have. An A1c test simply measures the percentage of red blood cells carrying that sugar coating.
The reason this test reflects a two-to-three-month window is straightforward: red blood cells live about three months. Once glucose attaches to hemoglobin, it stays there for the life of that cell. So at any given moment, your blood contains red blood cells of varying ages, and the sugar coating on each one is a record of what your blood sugar was doing when that cell was circulating. The test captures all of that at once, giving you a rolling average rather than a snapshot.
What the Results Mean
The National Institute of Diabetes and Digestive and Kidney Diseases breaks A1c results into three categories:
- 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
A diagnosis of diabetes requires confirmation with a second test unless you already have clear symptoms like excessive thirst, frequent urination, or unexplained weight loss. A single result at 6.5% doesn’t lock in a diagnosis on its own.
Your A1c percentage also translates to an estimated average glucose (eAG), which can make the number feel more concrete. An A1c of 6% corresponds to an average blood sugar of about 126 mg/dL. At 7%, it’s roughly 154 mg/dL. At 8%, about 183 mg/dL. The relationship is linear: each 1% increase in A1c adds roughly 29 mg/dL to your estimated average.
A1c vs. Daily Blood Sugar Checks
If you already check your blood sugar with a finger-stick meter or a continuous glucose monitor, you might wonder why you need an A1c test at all. The difference is scope. A finger stick tells you what your blood sugar is right now. A continuous monitor tracks it minute by minute. But the A1c captures your 24-hour average, including the times you’re least likely to test, like the hours after meals when blood sugar tends to spike.
This matters because people with diabetes tend to check more often when their sugar is low (before meals, first thing in the morning) and less often when it’s high. That skews the average from a meter downward. Your A1c doesn’t have that bias. It reflects every high and every low over the full three months, whether you were testing at that moment or not. That’s why your meter average and your A1c-derived average may not match, and your A1c is often considered the more reliable indicator of overall control.
How Often You Need the Test
If you have diabetes and your blood sugar is well controlled, the standard recommendation is at least twice a year. If your numbers aren’t where they need to be, or if you’ve recently changed medications or treatment plans, your doctor will likely want to check more frequently, typically every three months. For people without diabetes, A1c is used as a screening tool and may be ordered as part of routine bloodwork, especially if you have risk factors like obesity, family history of diabetes, or a previous prediabetes result.
What the Test Can and Can’t Tell You
The A1c test is used to both diagnose and monitor type 2 diabetes, and to monitor type 1 diabetes. But it can’t distinguish between them. Type 1 and type 2 diabetes have different causes and often require different treatments, so if you’re newly diagnosed, additional testing is needed to determine which type you have.
The test also can’t reveal day-to-day patterns. Two people can have the same A1c of 7% with very different daily experiences. One might have relatively stable blood sugar hovering around 154 mg/dL. The other might swing between 60 and 250 mg/dL throughout the day, with the highs and lows averaging out to the same number. For managing diabetes day to day, you still need real-time monitoring to catch those swings.
When Results May Be Inaccurate
Several conditions can throw off A1c results, and they all relate to the same basic principle: the test assumes your red blood cells have a normal lifespan and a normal hemoglobin structure. When either of those assumptions breaks down, the number becomes less reliable.
Conditions that shorten the lifespan of red blood cells, like hemolytic anemia or recovery from significant blood loss, will make your A1c falsely low. Your red blood cells are dying off faster than normal, so they have less time to accumulate glucose. The test reads a lower percentage than your actual blood sugar control would suggest.
Iron deficiency anemia works in the opposite direction. It’s associated with falsely elevated A1c results. This is particularly relevant during late pregnancy, when iron deficiency is common. In fact, doctors are generally cautious about using A1c for diagnosis during the second and third trimesters of pregnancy for this reason.
Inherited hemoglobin variants, such as sickle cell trait or hemoglobin C trait, can also affect accuracy depending on the specific lab method used. People with sickle cell disease face additional complications because the disease itself causes anemia, faster red blood cell turnover, and sometimes requires transfusions, all of which make A1c unreliable as a long-term glucose marker. Chronic kidney failure can also cause A1c to underestimate true blood sugar levels, particularly in patients on dialysis.
If any of these conditions apply to you, alternative tests like fructosamine or glycated albumin may give a more accurate picture of blood sugar control. These tests measure glucose attachment to different proteins and aren’t affected by red blood cell lifespan.
No Special Preparation Needed
One of the practical advantages of the A1c test is that it doesn’t require fasting. You can eat and drink normally before your appointment. That said, your doctor may order other blood tests at the same time, like a cholesterol panel, that do require fasting. If you’re unsure whether to eat beforehand, check with your doctor’s office when you schedule the draw.