Glycohemoglobin

Alternative names 
Glycosylated hemoglobin; Hemoglobin - glycosylated; A1C; GHb; HbA1c; Diabetic control index

Definition
HbA1c is a test that measures the amount of glycosylated hemoglobin in your blood. The test gives a good estimate of how well diabetes is being managed over time.

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
No special preparation in necessary. You do not need to fast.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experience, and level of trust. For general information regarding how you can prepare your child, see the following:

     
  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

This test measures blood sugar control over an extended period in people with diabetes. In general, the higher your HbA1c value, the higher the risk that you will develop complications from diabetes (eye disease, kidney disease, nerve damage, heart disease, and stroke). This is especially true if your HbA1c remains elevated on more than one occasion.

The closer your HbA1c value is to normal, the less risk you have for these complications.

Normally, only a small percentage of the hemoglobin (Hb) molecules in red blood cells become glycosylated (that is, chemically linked to glucose). The percent of glycosylation increases over time, and is higher if there is more glucose in your blood. Therefore, older red blood cells will have a greater percent of glycosylated hemoglobin, and diabetics whose blood glucose has been too high will have a greater percent of glycosylated hemoglobin.

Normal Values
HbA1c is normal if it is 5% or less. Normal ranges may vary slightly depending on the laboratory used.

What abnormal results mean

The test can show that your blood glucose levels have not been well-regulated over a period of weeks to months. If the HbA1c value is above 7%, it means your diabetes is poorly controlled. High values mean you are at greater risk of diabetic complications. If you can bring your level down, you decrease your chances of long-term complications.

You should try to keep your level below 7%. Ask your doctor how often you should have your Hb A1c tested (usually every 3 or 6 months is recommended).

What the risks are

Risks related to getting your blood drawn include:

     
  • Excessive bleeding  
  • Fainting or feeling lightheaded  
  • Hematoma (blood accumulating under the skin)  
  • Infection (a slight risk any time the skin is broken)  
  • Multiple punctures to locate veins Special considerations Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

    Johns Hopkins patient information

    Last revised: December 7, 2012
    by Mamikon Bozoyan, M.D.

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