hemoglobin test

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Hemoglobin Test



Hemoglobin is a protein inside red blood cells that carries oxygen throughout the body. A hemoglobin test reveals how much hemoglobin is in a person's blood, helping to diagnose and monitor anemia and polycythemia vera.


A hemoglobin test is done when a person is ill or during a general physical examination. Good health requires an adequate amount of hemoglobin. The amount of oxygen in the body tissues depends on how much hemoglobin is in the red cells. Without enough hemoglobin, the tissues lack oxygen and the heart and lungs must work harder to try to compensate.
If the test indicates a "less than" or "greater than" normal amount of hemoglobin, the cause of the decrease or increase must be discovered. A low hemoglobin usually means the person has anemia. Anemia results from conditions that decrease the number or size of red cells, such as excessive bleeding, a dietary deficiency, destruction of cells because of a transfusion reaction or mechanical heart valve, or an abnormally formed hemoglobin.
A high hemoglobin may be caused by polycythemia vera, a disease in which too many red blood cells are made.
Hemoglobin levels also help determine if a person needs a blood transfusion. Usually a person's hemoglobin must be below 8 gm/dl before a transfusion is considered.


Hemoglobin is made of heme, an iron compound, and globin, a protein. The iron gives blood its red color. Hemoglobin tests make use of this red color. A chemical is added to a sample of blood to make the red blood cells burst. When they burst, the red cells release hemoglobin into the surrounding fluid, coloring it clear red. By measuring the color using an instrument called a spectrophotometer, the amount of hemoglobin is determined.
Hemoglobin is often ordered as part of a complete blood count (CBC), a test that includes other blood cell measurements.
Some people inherit hemoglobin with an abnormal structure. These abnormal hemoglobins cause diseases, such as sickle cell or Hemoglobin C disease. Special tests, using a process called hemoglobin electrophoresis, identify abnormal hemoglobins.


This test requires 5 mL of blood. 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 that vein. Vacuum action draws the blood through the needle into an attached tube. Collection of the sample takes only a few minutes.
The person should avoid smoking before this test as smoking can increase hemoglobin levels.


Discomfort or bruising may occur at the puncture site or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs to the puncture site relieve discomfort.

Normal results

Normal values vary with age and sex. Women generally have lower hemoglobin values than men. Men have 14.0-18.0 g/dL, while women have levels of 12.0-16.0 g/dL.

Abnormal results

A low hemoglobin usually indicates the person has anemia. Further tests are done to discover the cause and type of anemia. Dangerously low hemoglobin levels put a person at risk of a heart attack, congestive heart failure, or stroke.
A high hemoglobin indicates the body is making too many red cells. Further tests are done to see if this is caused by polycythemia vera, or as a reaction to illness, high altitudes, heart failure, or lung disease.
Fluid volume in the blood affects hemoglobin values. Pregnant women and people with cirrhosis have extra fluid, which dilutes the blood, decreasing the hemoglobin. Dehydration concentrates the blood, increasing the hemoglobin.



Hsia, Connie C. W. "Respiratory Function of Hemoglobin." New England Journal of Medicine 338 (January 1998): 239-247.

Key terms

Anemia — A condition characterized by a decrease in the size or number of red blood cells.
Hemoglobin — A protein inside red blood cells that carries oxygen to body tissues.
Polycythemia vera — A disease in which the bone marrow makes too many red blood cells.

hemoglobin (Hb, Hgb) test

a blood test that measures the total amount of hemoglobin in the peripheral blood, which reflects the number of red blood cells in the blood. The test is normally performed as part of a complete blood count. Abnormal levels indicate anemia, erythrocytosis, and sickle cell disease, among others.
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