glycosylated hemoglobin test


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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 test

 [gli-ko´sĭ-lāt″ed]
measurement of the percentage of hemoglobin A molecules that have formed a stable ketoamine linkage between the hemoglobin and glucose; this can be used to assess the control of blood glucose levels. In persons with a normal blood glucose level it amounts to about 7 per cent of the total; in those with diabetes mellitus it is about 14.5 per cent. The higher the blood glucose levels have been over the previous three months, the higher the glycosylated hemoglobin will be.

glycosylated hemoglobin (GHb, GHB) test

a blood test used to monitor diabetes treatment. It measures the amount of hemoglobin A1c in the blood and provides an accurate long-term index of the patient's average blood glucose level.

hemoglobin

an allosteric protein found in erythrocytes that transports molecular oxygen (O2) in the blood. Symbol Hb.
Oxygenated hemoglobin (oxyhemoglobin) is bright red in color; hemoglobin unbound to oxygen (deoxyhemoglobin) is darker. This accounts for the bright red color of arterial blood, in which the hemoglobin is about 97% saturated with oxygen. Venous blood is darker because it is only about 20-70% saturated, depending on how much oxygen is being used by the tissues.
The affinity of hemoglobin for carbon monoxide is 210 times as strong as its affinity for oxygen. The complex formed (carboxyhemoglobin) cannot transport oxygen. Thus, carbon monoxide poisoning results in hypoxia and asphyxiation.
Another form of hemoglobin that cannot transport oxygen is methemoglobin, in which the iron atom is oxidized to the +3 oxidation state. During the life span of a red cell, hemoglobin is slowly oxidized to methemoglobin. At least four different enzyme systems can convert methemoglobin back to hemoglobin. When these are defective or overloaded, methemoglobinemia, in which high methemoglobin levels cause dyspnea and cyanosis, may result.
As red cells wear out or are damaged, they are ingested by macrophages of the reticuloendothelial system. The porphyrin ring of heme is converted to the bile pigment bilirubin, which is excreted by the liver. The iron is transported to the bone marrow to be incorporated in the hemoglobin of newly formed erythrocytes.

hemoglobin A1c
see glycosylated hemoglobin (below).
carcass hemoglobin tests
include hemoglobin extraction test, hemoglobin-pseudoperoxidase test. Used on suspect meat to determine if it has been properly bled out; poor bleeding is an indication of fever or septicemia.
hemoglobin concentration
varies with the hematocrit; determined by several methods. Assesses the oxygen-carrying capacity of blood.
cyanotic hemoglobin malformations
insufficient oxygenated hemoglobin is received in the peripheral capillary beds resulting in blue discoloration of tissues, and an incapacity of the body to maintain a life-sustaining level of activity.
glycosylated hemoglobin
hemoglobin A with a glucose moiety attached to the amino terminal valine of the beta chain. This type of hemoglobin is made at a slow constant rate during the life span of the erythrocyte. Increased levels correlate with glucose intolerance in diabetes. With adequate insulin treatment, levels return to the normal range and periodic assays can be helpful in evaluating effective control of diabetes mellitus. Called also hemoglobin A1c.
glycosylated hemoglobin test
measurement of the percentage of hemoglobin A molecules that formed a stable ketoamine linkage between the amino terminal valine residue of the beta chain and a glucose moiety; used to assess diabetic control.
hemoglobin-oxygen dissociation curve
the incremental increase in oxygen saturation of the hemoglobin with each unit increase of the partial pressure of oxygen in the blood. Any factor that shifts the curve to the right will automatically reduce the concentration of O2 held by the hemoglobin and increase the rate of its delivery to tissues.
urine hemoglobin
hemoglobin variants
the globin part of hemoglobin is composed of a large number of amino acids and the hemoglobins are therefore susceptible to a great many variations. In humans a large number of variants have been identified but only a few in animals and none are deleterious. The identified ones are the three adult hemoglobin types, HbA, HbB and HbC. There is also an embryonic, HbE, and a fetal hemoglobin, HbF.
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