gly·co·sy·lat·ed hemoglobin (gl -k  s -l  t d)n. Any of four hemoglobin fractions that together account for less than 4 percent of the total hemoglobin in the blood. |
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.
Mentioned in: Glycosylated Hemoglobin Test
glycosylated hemoglobin (GHb/Hb A1c)
[glīkō′silā′tid]
a hemoglobin A molecule with a glucose group on the N-terminal valine amino acid unit of the beta chain. The glycosylated hemoglobin concentration represents the average blood glucose level over the previous several weeks. In controlled diabetes mellitus the concentration of glycosylated hemoglobin is within the normal range, but in uncontrolled cases the level may be three to four times the normal concentration. Assays of Hb A1c, which normally has a 4-month life span, reveal whether glucose levels have been properly controlled during a period of several weeks before the test. The normal range is 1.8% to 4.0% for children; 2.2% to 4.8% for adults. Also spelled glycosylated haemoglobin.
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
see hemoglobinuria.
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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