What Is an A1C Test?

By Zachary T. Bloomgarden, MD, FACE

When a person with diabetes sees an endocrinologist [en-doh-cri-NA-lo-jist], they soon learn about a test called hemoglobin A1C, or HbA1c, or, simply, A1c. The A1C, they are told, should be as low as possible (the American Association of Clinical Endocrinologists [AACE] recommends a level of 6.5% or less). A person with diabetes may also learn that an A1C of 6.5% is a benchmark for diagnosing diabetes.

What is A1C? Is it truly useful? What does it mean as a benchmark of diabetes control?

A1C represents the attachment of glucose (sugar) to hemoglobin (the oxygen-carrying protein in our red blood cells). The red blood cells need glucose for their metabolism. When the hemoglobin in the red blood cell meets glucose, glucose slowly (over days and weeks) attaches to an amino acid on the hemoglobin. At this time, a person’s A1C level would show the amount of glucose that the red blood cells have been exposed to over time. Since the average life of a red blood cell is 3 to 4 months, the A1C shows an average blood sugar level, not just at the time the blood test was done, but during the long period leading up to that time.

Normally, when a person does not have diabetes, their blood sugar is below 100 mg/dL before meals, and it rarely rises over 120-130 mg/dL after meals. In these circumstances, the A1C is around 5%. This means that 5% of the hemoglobin molecules in that person’s millions of red blood cells have glucose attached. In mild diabetes (with a fasting blood glucose just over 125 mg/dL or the blood sugar 2 hours after an oral glucose tolerance test around 200 mg/dL), the A1C will be over 6%. So, 6% of their hemoglobin molecules have glucose attached. At 7%, 8%, and 10% the blood glucose levels during the day become higher, and so, blood sugar levels in the morning before eating might be around 150 with an A1C of 7%, then 180 with an A1C of 8%, and then 240 mg/dL with an A1C of 10%.

But this leads to an important issue. Not all people have red blood cells that live for the same amount of time. Men’s red blood cells live for about 117 days, and women’s red blood cells live for about 106 days. If one person’s red blood cells live for 4 months, and another person’s red blood cells live for 2 months, the A1C could be only half as great in the second person. It’s not quite that bad usually, but anemia, low red blood cell levels, certain genetic issues, and many illnesses, such as chronic infections and chronic kidney disease, lead to a lower blood A1C level. African American people tend to have higher A1C levels than white people, and young people have lower A1C levels than older people. Everyone’s body is different, so if we very carefully measure a person’s blood sugar many times a day for many days some people have a higher A1C and some have a lower A1C. Another factor in the different A1C levels is that lab tests are not perfect. Doctors expect a small range in values in most blood tests. For example, for an A1C blood test result of 6.5%, the person’s actual level could be anywhere between 6.3% and 6.7%.

A1C is, then, a very useful indicator of the average blood sugar level for a person with diabetes. It also may determine that a person is at risk of having diabetes. However, doctors look at it along with information about the average blood sugar levels for that specific person. This is why most endocrinologists advise that people with diabetes measure their own blood sugar levels at home regularly, at various times of the day. This gives the patient and the doctor a better sense of the control (or lack of control) of the person’s diabetes. The A1C test then helps to provide a full picture of how well the diabetes is controlled. The American Association of Clinical Endocrinologists (AACE) has recommended that though it makes sense to use A1C to diagnose diabetes, the test can be misleading. So, if a doctor finds that a patient’s A1C level is 6.5%, AACE terms it diagnostic of diabetes. AACE does recommend confirming this with the blood sugar reading “when feasible,” although my opinion is that blood sugar measurements should always be done. Certainly, if the patient’s diabetes status is not clear, the patient should have an oral glucose tolerance test. If the fasting glucose is 126 mg/dL or more or the 2-hour glucose is 200 mg/dL or more, then diabetes is confirmed. This approach, patients checking their own blood sugar (for diagnosed diabetes) and careful glucose testing (to diagnose diabetes), allows us to determine the diabetes condition of each person.

Dr. Zachary Bloomgarden has a national reputation for writing and lecturing on diabetes. He is Clinical Professor in the Department of Medicine and is active in the Division of Endocrinology & Metabolism of the Mount Sinai Medical Center, New York, New York. He runs a major teaching activity; the weekly diabetes clinic conferences with the participation of speakers from the institution and numerous invited guests; and participates in teaching rounds in the Departments of Geriatrics and Pediatrics, as well as Medicine.