More than 34 million Americans have diabetes, meaning their bodies either do not produce insulin or can’t use it properly. (Insulin is a hormone that helps the cells in our bodies take up glucose [sugar] in our blood, which we use for energy.)

Over time, uncontrolled diabetes can lead to a host of serious health issues, so it’s important for those with diabetes to keep their blood sugar levels in a healthy range—meaning it’s neither too low nor too high.

A hemoglobin A1C (also known as a hemoglobin A1C test or an HbA1c test) is a blood test that can provide information about average blood sugar levels. The test not only helps diabetes patients know if they are managing their blood sugar levels well; it also helps doctors provide a diagnosis of diabetes or prediabetes to those who are unaware they have the condition.

The Centers for Disease Control and Prevention (CDC) recommends that all adults ages 45 and older should receive a hemoglobin A1C test to assess their risk of prediabetes or diabetes. The test may also be administered to overweight people with diabetes risk factors who are younger than 45.

People of any age with diabetes risk factors whose test results are in the normal range should be tested again every three years. Those who are found to have prediabetes should be retested every one to two years.

Those diagnosed with diabetes during a screening hemoglobin A1C test should work with their doctors to manage the chronic condition. They’ll need to have their blood sugar levels checked with a hemoglobin A1C test every 3 to 6 months, depending on how well they’re able to control their diabetes.

“The A1C test is a great way for physicians to get an initial sense of someone's average glucose level over the three months before the test,” says Yale Medicine endocrinologist Raimund Herzog, MD, MHS. “In addition, it provides us with a great way to monitor treatment effects and progress over time.”

Hemoglobin A1C is a blood test that provides doctors with information about a person’s blood sugar levels. Interestingly, although a patient may provide a blood sample on a single day, the A1C test doesn’t provide details about their blood sugar levels on that particular day. Rather, the test gives doctors insights into the overall state of their blood sugar levels during the three-month period leading up to the test. This is possible because red blood cells live for an average of three months.

Red blood cells contain a protein called hemoglobin, which gives blood its red color. Hemoglobin is responsible for carrying oxygen throughout the body, from the lungs to other organs and body parts. Another substance, blood sugar (glucose), which comes from food sources, also travels through the bloodstream.

When blood sugar encounters red blood cells, it attaches itself to hemoglobin, where it remains. Once linked, glucose and hemoglobin become what’s called “glycated hemoglobin,” also known as A1C. The hemoglobin A1C test measures the percentage of red blood cells in the bloodstream that have glucose attached to hemoglobin.

Thus, the hemoglobin A1C test results show how much glycated hemoglobin has built up in a person’s bloodstream during the previous three months. A higher percentage of red blood cells containing glycated hemoglobin means that a person has higher levels of blood sugar in their bloodstream. For people with diabetes, this means the disease is not well-controlled. For those who have not been diagnosed before, this may mean that they have prediabetes or diabetes.

All people have some glycated hemoglobin within their red blood cells, and a lower percentage is considered normal.

People may have their blood drawn by a nurse at the doctor’s office or by a phlebotomist at a lab. It’s also possible to use an at-home finger prick test, but a blood draw by professionals is the preferred method for the hemoglobin A1C test. After the blood is drawn, a lab will determine the test results and share that information with doctors and patients.

When A1C test results are:

• Lower than 5.7%, that is considered “normal”
• Between 5.7% and 6.4%, that indicates a diagnosis of prediabetes
• 6.5% or higher, that means a person has diabetes

Doctors recommend that people with diabetes should strive to keep their A1C levels below 7%. Together, doctors and patients may achieve this goal with a plan that includes dietary changes, weight loss, exercise, and/or medication.

If you have diabetes, the A1C test is the best way to check your average blood sugar levels over a three-month period to gauge whether or not you are managing your condition effectively.

There are other blood tests that may be used to diagnose prediabetes or diabetes, including:

• A fasting blood sugar test, for which you must fast overnight before having your blood drawn
• A glucose tolerance test, for which you must drink a sweet liquid containing glucose, then wait in the doctor’s office for a set time period before having your blood drawn
• A random blood sugar test, which doesn’t require any special preparation, but it only screens for diabetes.

Doctors may use one or more of these tests to diagnose prediabetes or diabetes. Although the A1C test is effective, people with certain health conditions may not get accurate results from it. Thus, another test should be used to confirm a diabetes or prediabetes diagnosis in those cases.

Hemoglobin A1C test results may not be accurate when people have health conditions such as:

• Sickle cell anemia
• Thalassemia
• Anemia
• Kidney failure
• Liver disease, including cirrhosis
• Enlarged spleen
• Blood transfusions
• Blood hemorrhage
• High triglyceride levels
• Organ transplantation
• Pregnancy
• Alcohol use disorder
• Vitamin B12 deficiency
• Jaundice
• Inherited blood disorders, such as spherocytosis

Certain medications or dietary supplements may also affect the test, giving people inaccurate results. They include:

• Opioids
• Certain drugs that are used to treat HIV
• Iron supplements
• Vitamin C supplements

To perform a hemoglobin A1C test, a sample of blood must be given, either by finger prick or simple blood draw. Very few risks are associated with these blood-sampling methods. A bit of short-lived pain from the needle pricking the skin is normal and expected. Some people may develop a bruise at the site where the needle was inserted.

Visit the Yale Medicine Diabetes Content Center for more diabetes-related articles and videos.