Glycosylated Hemoglobin Test
Glycosylated Hemoglobin Test
Definition
Glycosylated hemoglobin is a test that indicates how much sugar has been in a person's blood during the past two to four months. It is used to monitor the effectiveness of diabetes treatment.
Purpose
Diabetes is a disease in which a person cannot effectively use sugar in the blood. Left untreated, blood sugar levels can be very high. High sugar levels increase risk of complications, such as damage to eyes, kidneys, heart, nerves, blood vessels, and other organs.
A routine blood sugar test reveals how close to normal a sugar level is at the time of the test. The glycosylated hemoglobin test reveals how close to normal it has been during the past several months.
This information helps a physician evaluate how well a person is responding to diabetes treatment and to determine how long sugar levels have been high in a person newly diagnosed with diabetes.
Description
The Diabetes Control and Complications Trial (DCCT) demonstrated that people with diabetes who maintained blood glucose (sugar) and total fasting hemoglobin levels at or close to a normal range decreased their risk of complications by 50-75%. Based on results of this study, the American Diabetes Association (ADA) recommends routine glycosylated hemoglobin testing to measure long-term control of blood sugar.
Glycosylated hemoglobin measures the percentage of hemoglobin bound to glucose. Hemoglobin is a protein found in every red blood cell. As hemoglobin and glucose are together in the red blood cell, the glucose gradually binds to the A1c form of hemoglobin in a process called glycosylation. The amount bound reflects how much glucose has been in the blood during the past average 120-day lifespan of red cells.
Several methods are used to measure the amount of bound hemoglobin and glucose. They are electrophoresis, chromatography, and immunoassay. All are based on the separation of hemoglobin bound to glucose from that without glucose.
The ADA recommends glycosylated hemoglobin be done during a person's first diabetes evaluation, again after treatment is begun and sugar levels are stabilized, then repeated at least semiannually. If the person does not meet treatment goals or sugar levels have not stabilized, the test should be repeated quarterly.
Other names for the test include: Hemoglobin A1c, Diabetic control index, GHb, glycosylated hemoglobin, and glycated hemoglobin. The test is covered by insurance. Results usually are available the following day.
Preparation
A person does not need to fast before this test. A healthcare worker ties a tourniquet on the person's upper arm, locates a vein in the inner elbow region, and inserts a needle into the vein. Vacuum action draws the blood through the needle into an attached tube. Collection of the sample takes only a few minutes. This test requires 5 mL of blood.
In 2004, more convenient fingerstick methods of the test were being developed. An over-the-counter, at-home test kit was in the beginning stages of approval. People with diabetes could stick their own fingers, draw the blood and send the sample by mail for results within five to seven days.
Aftercare
Discomfort or bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure to the puncture site until bleeding stops reduces bruising. Warm packs relieve discomfort.
Normal results
Diabetes treatment should achieve glycosylated hemoglobin levels of less than 7.0%. Normal value for a non-diabetic person is 4.0-6.0%.
Because laboratories use different methods, results from different laboratories can not always be compared. The National Glycosylation Standardization Program gives a certification to laboratories using tests standardized to those used in the DCCT study.
Abnormal results
Results require interpretation by a physician with knowledge of the person's clinical condition, as well as the test method used. Some methods give false high or low results if the person has an abnormal hemoglobin, such as hemoglobin S or F.
KEY TERMS
Diabetes mellitus— A disease in which a person can't effectively use sugar in the blood to meet the needs of the body. It is caused by a lack of the hormone insulin.
Glucose— The main form of sugar used by the body for energy.
Glycosylated hemoglobin— A test that measures the amount of hemoglobin bound to glucose. It is a measure of how much glucose has been in the blood during the past two to four months.
Conditions that increase the lifespan of red cells, such as a splenectomy (removal of the spleen), falsely increase levels. Conditions that decrease the lifespan, such as hemolysis (disruption of the red blood cell membrane), falsely decrease levels.
Resources
PERIODICALS
"Simple Choice A1c." Diabetes Forecast January 2004: RG7.
ORGANIZATIONS
American Diabetes Association. 1701 North Beauregard Street, Alexandria, VA 22311. (800) 342-2383. 〈http://www.diabetes.org〉.
Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. 〈http://www.cdc.gov〉.
National Diabetes Information Clearinghouse. 1 Information Way, Bethesda, MD 20892-3560. (800) 860-8747. 〈http://www.niddk.nih.gov/health/diabetes/ndic.htm〉.
Glycosylated Hemoglobin
GLYCOSYLATED HEMOGLOBIN
The red blood cells of all individuals contain hemoglobin, which is responsible for carrying oxygen through the bloodstream. When hemoglobin combines with glucose (sugar), a molecule called glycosylated hemoglobin, or Hemoglobin A1c (HgbA1c), is formed. Since everyone has glucose in their blood, all individuals also have glycosylated hemoglobin in their blood (usually between 3 and 5 percent of blood).
The amount of A1c in red blood cells is proportional to the amount or concentration of glucose in the blood, and to the age of the red blood cells. (The average red blood cell lives approximately 120 days, with new ones replacing dying red blood cells continuously. Accordingly, healthy individuals have a mixture of "young" and "old" red blood cells at all times.)
If an individual has high blood-sugar levels, such as exists in poorly controlled or untreated diabetes mellitus, the glycosylated hemoglobin percentage will be elevated. Since it is also related to the "age" of the red blood cells, the glycosylated hemoglobin percentage will correspond to the average glucose level over the previous two to three months. This average level is in contrast to a routine measurement of the blood-sugar level, which reflects any food intake over the previous twelve hours.
CLINICAL USEFULNESS
In diabetes mellitus, the blood sugar is elevated in the fasting state, with levels exceeding 126 milligrams per deciliter (mg/dL). Persistently elevated blood-glucose levels result in many chronic complications, such as kidney failure, blindness, and poor circulation in the legs, which can result in amputation. Accordingly, it is recommended that diabetics keep their blood-sugar level in the normal range whenever possible. This can be achieved through diet, exercise, and medications. The American Diabetes Association (ADA), recommends that all health care clinicians routinely obtain glycosylated hemoglobin levels in all diabetics. In 2001, the ADA recommended that this testing be performed at least twice annually for those diabetics whose blood sugar is well-controlled, and more frequently in those with persistently elevated blood sugars. Just as cholesterol levels are used to predict the risk of developing heart disease, the glycosylated hemoglobin value can predict the risk of developing many of the chronic complications associated with diabetes.
OTHER USES
Researchers collecting information on large numbers of individuals can assist scientists in determining whether relationships exist between glycosylated hemoglobin levels and age, gender, ethnicity, or socioeconomic status.
Research directed at the relationship between high glycosylated hemoglobin levels and various disease states could assist health care providers to predict the presence or absence of disease. For public health officials, the determination of glucosylated hemoglobin levels for community areas could provide them with information to develop community-based interventions to improve the control of blood-sugar levels in communities and neighborhoods with elevated average glycosylated levels.
Patrick Dowling
Michelle Anne Bholat
(see also: Diabetis Mellitus; Screening )
Bibliography
American Diabetes Association (2001). "Clinical Practice Recommendations 2001." Diabetes Care 24(Supp.1):1–7.