A hemoglobin A1c test is a blood test that measures your average blood sugar level over the past two to three months. It’s one of the primary tools used to diagnose diabetes and prediabetes, and it’s also how people with diabetes track how well their blood sugar is being managed over time. Unlike a standard blood sugar check, which captures a single moment, the A1c gives a longer view of what’s been happening in your body.
How the Test Works
Hemoglobin is a protein inside your red blood cells that carries oxygen. When sugar (glucose) circulates in your bloodstream, some of it 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.
The reason this test reflects roughly three months of blood sugar history comes down to the lifespan of red blood cells. Each red blood cell lives about three months before your body replaces it. Once glucose sticks to hemoglobin, it stays there for the life of that cell. So at any given time, your blood contains a mix of newer and older red blood cells, and the amount of sugar-coated hemoglobin across all of them reflects your average blood sugar over that entire window.
What the Numbers Mean
The result comes back as a percentage. Higher percentages mean higher average blood sugar. The American Diabetes Association uses these thresholds:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
Prediabetes means your blood sugar levels are elevated but haven’t reached the diabetes range. It’s a signal that your body is having more difficulty processing sugar than it should, and it’s the stage where lifestyle changes can make the biggest difference in preventing progression to type 2 diabetes.
A diabetes diagnosis based on A1c typically requires confirmation, either by repeating the A1c test or through a different blood sugar test. A single result at 6.5% wouldn’t usually lead to a diagnosis on its own unless you’re also showing symptoms like increased thirst, frequent urination, or unexplained weight loss.
Translating A1c to Daily Blood Sugar
If you also check your blood sugar with a glucose meter, it helps to understand what your A1c percentage looks like in those daily numbers. The conversion follows a formula: multiply the A1c percentage by 28.7, then subtract 46.7. That gives you an estimated average glucose in mg/dL.
Here’s what that looks like in practice:
- A1c of 6%: Average blood sugar around 126 mg/dL
- A1c of 7%: Average blood sugar around 154 mg/dL
- A1c of 8%: Average blood sugar around 183 mg/dL
- A1c of 9%: Average blood sugar around 212 mg/dL
- A1c of 10%: Average blood sugar around 240 mg/dL
Keep in mind this is an average. Two people with the same A1c could have very different daily patterns. One might have steady glucose levels, while another swings between highs and lows that happen to average out to the same number.
A1c Targets for People With Diabetes
For most nonpregnant adults with diabetes, the general target is an A1c below 7%. Reaching and maintaining that level significantly reduces the risk of complications affecting the eyes, kidneys, and nerves. Some people may aim even lower if they can do so safely without frequent episodes of low blood sugar.
Not everyone has the same goal, though. An A1c target of up to 8% may be more appropriate for older adults dealing with multiple chronic conditions, people with limited life expectancy, or anyone who experiences dangerous drops in blood sugar when treatment is too aggressive. The target is meant to be personalized. Factors like how long someone has had diabetes, their age, whether they have heart disease, and how well they recognize symptoms of low blood sugar all factor into what makes sense.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends screening for prediabetes and type 2 diabetes in adults aged 35 to 70 who are overweight or obese. If results come back normal, repeating the test every three years is a reasonable schedule. People with risk factors like a family history of diabetes, a history of gestational diabetes, or belonging to a racial or ethnic group with higher diabetes rates may benefit from earlier or more frequent screening.
For people already diagnosed with diabetes, A1c is typically checked two to four times per year, depending on how stable blood sugar control has been and whether treatment has recently changed.
No Fasting Required
One of the practical advantages of the A1c test is that you don’t need to fast before it. You can eat and drink normally beforehand, which makes it easier to schedule and less disruptive than fasting glucose tests. That said, your doctor may order other blood work at the same time (like a cholesterol panel) that does require fasting, so it’s worth asking ahead of time.
What Can Throw Off Your Results
Because the test depends on red blood cells behaving normally, anything that changes how long your red blood cells live or how hemoglobin functions can skew the result. Conditions that shorten the lifespan of red blood cells, like sickle cell disease or other hemoglobin variants, can make A1c results unreliable, often reading lower than your true average blood sugar. Significant blood loss or blood transfusions can have a similar effect. Certain medications, including some opioids and HIV treatments, can also push results higher or lower than expected.
Iron deficiency anemia tends to falsely raise A1c readings, while kidney disease can affect results in either direction depending on the specifics. If you have any of these conditions, your doctor may rely on alternative tests like fructosamine (which measures a shorter window of blood sugar) or continuous glucose monitoring to get a more accurate picture.
Lab Tests vs. In-Office Finger Pricks
A1c can be measured two ways: through a standard blood draw sent to a laboratory, or with a point-of-care device that uses a finger prick and returns results in minutes. Lab-based tests are highly accurate, with most methods showing less than 2.5% variability between laboratories and many performing even better than that.
Point-of-care devices are convenient but less precise. A large review of 13 commonly used devices found that all of them showed some degree of bias compared to lab results, with most reading slightly lower than the true value. A device that consistently reads 0.5% too high, for example, would flag someone with a true A1c of 6.0% as having 6.5%, potentially leading to a diabetes diagnosis that isn’t warranted. A device that reads 0.5% too low would miss people who genuinely have diabetes.
In-office finger-prick A1c tests are useful for monitoring trends, but a diagnosis of diabetes or prediabetes should be confirmed with a lab-drawn test. In the U.S., facilities using these quick-test devices aren’t required to participate in proficiency testing, meaning there’s no external check on whether their results are accurate.