A1C is a blood test that measures the percentage of your red blood cells coated with glucose (sugar). It gives you a snapshot of your average blood sugar over the past two to three months, rather than just the moment your blood is drawn. Doctors use it to screen for diabetes, diagnose prediabetes, and monitor how well blood sugar is being managed over time.
How the A1C Test Works
Your red blood cells contain hemoglobin, a protein that carries oxygen throughout your body. Glucose in your bloodstream naturally sticks to hemoglobin in a process called glycation. The higher your blood sugar runs, the more hemoglobin gets coated with glucose. An A1C test simply measures what percentage of your hemoglobin has glucose attached to it.
The reason this works as a long-term average, rather than a point-in-time reading, comes down to how long red blood cells live. Red blood cells survive about 100 days on average before your body replaces them. Glucose stays attached to hemoglobin for the entire life of the cell, so the test captures a rolling record of your blood sugar levels over roughly three months. More recent weeks are weighted more heavily, since newer red blood cells make up a larger share of the total at any given time.
Unlike a fasting glucose test, A1C doesn’t require any preparation. You can eat and drink normally before the blood draw.
What the Numbers Mean
The result comes back as a percentage. The American Diabetes Association uses these thresholds:
- Below 5.7%: Normal blood sugar regulation
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is higher than normal but hasn’t reached diabetes levels
- 6.5% or higher: Diabetes
Prediabetes is a signal that your body is losing its ability to manage glucose efficiently. It doesn’t guarantee you’ll develop diabetes, but it does mean the risk is elevated and lifestyle changes can make a real difference at this stage.
If you want to connect your A1C to the kind of blood sugar number you’d see on a glucose meter, there’s a simple conversion. Multiply the A1C by 28.7, then subtract 46.7. So 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. This formula, developed by the A1C-Derived Average Glucose (ADAG) Study Group, is what appears on many lab reports as your “estimated average glucose” or eAG.
Why A1C Matters for Complications
A1C isn’t just an abstract number. It’s closely tied to the risk of the damage diabetes does over time, particularly to small blood vessels in the eyes, kidneys, and nerves. Research published in Diabetes Care found that the risk of these microvascular complications rises significantly once A1C crosses about 7.2%. At that level, the risk was roughly 32% higher compared to people in the 6.3% to 6.6% range. By the time A1C reaches 9.5% or above, the risk nearly doubles.
That same research found that pushing A1C below about 6.7% didn’t provide additional protective benefit, at least in people with kidney disease. This is part of why most treatment targets aim for an A1C under 7% rather than trying to drive it as low as possible, which can carry its own risks, including dangerous low blood sugar episodes.
How Often You Should Get Tested
Testing frequency depends on how well your blood sugar is controlled. Clinical guidelines recommend testing every three months for people whose diabetes is not well controlled or who have recently changed their treatment plan. If your blood sugar has been stable and on target, testing every six months is generally sufficient. Evidence suggests that for people already at their goal, testing every six months maintains blood sugar just as effectively as every three months.
For people without diabetes, A1C is often included as part of routine bloodwork during annual checkups, especially after age 45 or if you have risk factors like obesity, family history of diabetes, or a sedentary lifestyle.
When A1C Results Can Be Misleading
Because the test depends on red blood cells behaving normally, anything that changes how long your red blood cells live or how hemoglobin is structured can throw off the result.
Conditions that shorten red blood cell lifespan, like hemolytic anemia or recent significant blood loss, will falsely lower your A1C. Your red blood cells haven’t been around long enough to accumulate a representative amount of glucose, so the number looks better than your actual blood sugar control. On the other hand, iron deficiency anemia is associated with falsely elevated A1C readings, making blood sugar control appear worse than it is. This is also why A1C can read higher in late pregnancy, when iron deficiency is common.
Genetic hemoglobin variants, including sickle cell trait and hemoglobin C trait, can also affect accuracy depending on the lab method used. People with chronic kidney disease, particularly those on dialysis, may get unreliable A1C readings due to a combination of anemia, transfusions, and altered red blood cell turnover. In these cases, doctors may use alternative markers like glycated albumin, which measures glucose binding to a blood protein that turns over faster and isn’t affected by red blood cell issues.
If you have any of these conditions and your A1C doesn’t seem to match your day-to-day glucose readings, it’s worth discussing with your doctor whether an alternative test would give a more accurate picture.
A1C vs. Other Blood Sugar Tests
A1C fills a different role than fasting glucose or oral glucose tolerance tests. A fasting glucose test measures your blood sugar at a single point in time, after you haven’t eaten for at least eight hours. It’s useful, but it only captures one moment. You could have a normal fasting glucose and still run high after meals throughout the day.
A1C captures the full picture, including post-meal spikes, overnight levels, and everything in between. That’s why it’s become the preferred tool for monitoring ongoing diabetes management. It also doesn’t require fasting, which makes it more convenient and less likely to be affected by what you ate the night before or how stressed you were that morning.
For diagnosis, doctors sometimes use A1C alongside a fasting glucose test to confirm results, since each test has its own limitations. A single A1C reading at or above 6.5% is enough to diagnose diabetes, but a second test is typically done to confirm unless symptoms of high blood sugar are already obvious.