Glycohemoglobin (HbA1c, A1c)
Glycohemoglobin (A1c) is a blood test that checks the amount of sugar (glucose) bound to the hemoglobin in the red blood cells. When hemoglobin and glucose bond, a coat of sugar forms on the hemoglobin. That coat gets thicker when there's more sugar in the blood. A1c tests measure how thick that coat has been over the past 3 months, which is how long a red blood cell lives. People who have diabetes or other conditions that increase their blood glucose levels have more glycohemoglobin than normal.
An A1c test can be used to diagnose prediabetes or diabetes. The A1c test checks the long-term control of blood glucose levels in people with diabetes. Most doctors think checking an A1c level is the best way to check how well a person is controlling his or her diabetes.
A home blood glucose test measures the level of blood glucose only at that moment. Blood glucose levels change during the day for many reasons, including medicine, diet, exercise, and the level of insulin in the blood.
It is useful for a person who has diabetes to have information about the long-term control of blood sugar levels. The A1c test result does not change with any recent changes in diet, exercise, or medicines.
Glucose binds to hemoglobin in red blood cells at a steady rate. Since red blood cells last 3 to 4 months, the A1c test shows how much glucose is in the plasma part of blood. This test shows how well your diabetes has been controlled in the last 2 to 3 months and whether your diabetes treatment plan needs to be changed.
The A1c test can also help your doctor see how big your risk is of developing problems from diabetes, such as kidney failure, vision problems, and leg or foot numbness. Keeping your A1c level in your target range can lower your chance for problems.
Why It Is Done
This test is done to:
- Diagnose prediabetes and diabetes.
- Check your treatment for diabetes.
How To Prepare
You do not need to stop eating before you have an A1c test. This test can be done any time during the day, even after a meal.
How It Is Done
The health professional taking a sample of your blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
A1c is a blood test that checks the amount of sugar (glucose) bound to hemoglobin. The result is shown as a percentage. The result of your A1c test can also be used to estimate your average blood sugar level. This is called your estimated average glucose, or eAG. Your doctor will have your test results in a few days.
The American Diabetes Association (ADA) criteria to diagnose diabetes includes the option of testing A1c. The diagnosis of diabetes needs to be confirmed by repeating the same blood sugar test or doing a different test on another day.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Less than 5.7%
|Prediabetes (increased risk for diabetes)||
6.5% and higher
The ADA recommends that most nonpregnant adults who have type 1 or type 2 diabetes have an A1c level less than 7%.2 The American Academy of Pediatrics recommends that most children with type 2 diabetes have an A1c level less than 7%.3 Talk to your doctor about your diabetes treatment plan and your target A1c goal.
|A1c %||Estimated average plasma glucose (mg/dL)||Estimated average plasma glucose (mmol/L)|
|Children younger than 6 years old||Less than 8.5%|
|Children ages 6–12 years old||Less than 8%|
|Teens ages 13–19 years old||Less than 7.5%|
Some medical conditions can increase A1c levels, but the results may still be within a normal range. These conditions include Cushing's syndrome, pheochromocytoma, and polycystic ovary syndrome (PCOS).
Corticosteroid treatment increases the A1c level.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Having severe blood loss or a blood transfusion in the last 3 months.
- Having certain medical conditions, such as sickle cell anemia, hemolytic anemia, some types of thalassemia, and severe kidney disease.
- Having your spleen taken out. This changes the normal life cycle of red blood cells and A1c levels.
What To Think About
- If you have diabetes, your doctor may recommend that you have an A1c test every 3 to 6 months, depending on how well you are meeting your treatment goals.
- Some people who develop diabetes have normal A1c levels early in the course of their disease.
- A1c levels can be normal in some people who have untreated diabetes and certain medical conditions, such as sickle cell anemia, hemolytic anemia, severe kidney disease, or pregnancy.
- A1c levels are not useful for finding low blood sugar (hypoglycemia).
- The A1c test does not replace the need for other regular blood glucose tests, including checking your blood sugar at home and a regular blood glucose test.
- American Diabetes Association (2012). Diagnosis and classification of diabetes mellitus. Diabetes Care, 35(Suppl 1): S64–S71.
- American Diabetes Association (2013). Standards of medical care in diabetes—2013. Diabetes Care, 36(Suppl 1): S11–S66.
- American Academy of Pediatrics (2013). Clinical Practice Guideline: Management of newly diagnosed type 2 diabetes mellitus (T2DM) in children and adolescents. Pediatrics, 131(2): 364–382. Also available online: http://pediatrics.aappublications.org/content/131/2/364.full.html.
Other Works Consulted
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Specialist Medical Reviewer||David C.W. Lau, MD, PhD, FRCPC - Endocrinology|
|Last Revised||July 16, 2013|
Last Revised: July 16, 2013
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