Glycohemoglobin (HbA1c, A1c)

Glycohemoglobin (HbA1c, A1c)

Test Overview

A glycohemoglobin test, or hemoglobin 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 (sugar bound to hemoglobin) 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.
This test may not be appropriate for everyone because many things can affect the life span of red blood cells, such as the second or third trimester of pregnancy, a recent blood loss or a blood transfusion, sickle cell disease, hemodialysis, or erythropoietin (ESA) medicine.

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.

Risks

Blood test

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.

Results

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 include the option of testing A1c. Your doctor may repeat the test to confirm the diagnosis of diabetes.

Normal

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.

Hemoglobin
A1cfootnote 1
Normal

Less than 5.7%

Prediabetes (increased risk for diabetes)

5.7%–6.4%

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%.footnote 2 The ADA recommends that most children with type 1 diabetes have an A1c level less than 7.5%.footnote 3 The American Academy of Pediatrics recommends that most children with type 2 diabetes have an A1c level less than 7%.footnote 4 Talk to your doctor about your diabetes treatment plan and your target A1c goal.

A1c and estimated average glucose (eAG)footnote 2
A1c % Estimated average
plasma glucose (mg/dL)
Estimated average plasma glucose (mmol/L)
6%

126

7.0

7%

154

8.6

8%

183

10.2

9%

212

11.8

10%

240

13.4

11%

269

14.9

12%

298

16.5

High values

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.
  • Being on hemodialysis.
  • Taking erythropoietin (ESA) medicine.
  • Being in the second or third trimester of pregnancy.
  • 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.

References

Citations

  1. American Diabetes Association (2014). Diagnosis and classification of diabetes mellitus. Diabetes Care, 37(Suppl 1): S81–S90.

    DOI: 10.2337/dc14-S081. Accessed April 27, 2017.

  2. American Diabetes Association (2018). Standards of medical care in diabetes—2018. Diabetes Care, 41(Suppl 1): S1–S159. http://care.diabetesjournals.org/content/41/Supplement_1. Accessed December 8, 2017.
  3. Chiang JL, et al. (2014). Type 1 diabetes through the life span: A position statement of the American Diabetes Association. Diabetes Care, 37(7): 2034–2051. DOI: 10.2337/dc14-1140. Accessed July 21, 2014.
  4. 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.

Credits

ByHealthwise StaffPrimary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine Kathleen Romito, MD - Family Medicine Adam Husney, MD - Family Medicine Specialist Medical Reviewer Matthew I. Kim, MD - Endocrinology David C.W. Lau, MD, PhD, FRCPC - Endocrinology

Current as ofJanuary 29, 2018

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