Serum Hemoglobin
Alternate Names : Blood Hemoglobin, Serum Hemoglobin
Definition Serum hemoglobin is a test that measures the level of free (that is, outside red blood cells) hemoglobin in the blood.
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Why is the Test Performed?
The test is done to detect and monitor the severity of various kinds of hemolytic anemia, or breakdown of red blood cells.
Hemoglobin (Hb), the main component of red blood cells, is a protein that carries oxygen away from the lungs and carbon dioxide back to the lungs. A molecule of hemoglobin consists of 2 pairs of peptide (alpha and beta globins) chains and four heme groups, each with one atom of iron. At a normal oxygen tension of 100 mmHg in the blood vessels of the lungs, 95 - 98% of the Hb is combined with oxygen. In the tissues further from the lungs, where the oxygen tension is much lower, the oxygen readily separates from Hb.
Free plasma Hb separates into alpha-beta molecules. These are bound to a serum protein called haptoglobin and taken up by the liver. However, when the plasma Hb levels exceed 50 - 200 mg/dl, which is the capacity of haptoglobin to bind Hb, the free alpha-beta molecules pass into the urine.
Plasma Hb that is not bound to haptoglobin nor removed by the kidneys may be changed to a form called methemoglobin.
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