Hemoglobin is a protein inside red blood cells that carries oxygen. A hemoglobin test reveals how much hemoglobin is in a person's blood. This information can be used to help physician's diagnose and monitor anemia (a low hemoglobin level) and polycythemia vera (a high hemoglobin level).
A hemoglobin test is performed to determine the amount of hemoglobin in a person's red blood cells (RBCs). This is important because the amount of oxygen available to tissues depends upon how much oxygen is in the RBCs, and local perfusion of the tissues. Without sufficient hemoglobin, the tissues lack oxygen and the heart and lungs must work harder to compensate.
A low hemoglobin measurement usually means the person has anemia. Anemia results from a decrease in the number, size, or function of RBCs. Common causes include excessive bleeding, a deficiency of iron, vitamin B 12 , or folic acid, destruction of red cells by antibodies or mechanical trauma, and structurally abnormal hemoglobin. Hemoglobin levels are also decreased due to cancer, kidney diseases, other chronic diseases, and excessive IV fluids. An elevated hemoglobin may be caused by dehydration (decreased water), hypoxia (decreased oxygen), or polycythemia vera. Hypoxia may result from high altitudes, smoking, chronic obstructive lung diseases (such as emphysema), and congestive heart failure. Hemoglobin levels are also used to determine if a person needs a blood transfusion . Usually a person's hemoglobin must be below 7–8 g/dL before a transfusion is considered, or higher if the person has heart or lung disease. The hemoglobin concentration is also used to determine how many units of packed red blood cells should be transfused. A common rule of thumb is that each unit of red cells should increase the hemoglobin by approximately 1.0–1.5 g/dL.
Fluid volume in the blood affects hemoglobin values. Accordingly, the blood sample should not be taken from an arm receiving IV fluid. It should also be noted that pregnant women and people with cirrhosis, a type of permanent liver disease, have extra fluid, which dilutes the blood, decreasing the hemoglobin. Dehydration, a decreased amount of water in the body, concentrates the blood, which may cause an increased hemoglobin result.
A nurse or phlebotomist usually collects the sample by inserting a needle into a vein, or venipuncture, after cleaning the skin, which helps prevent infections.
Hemoglobin is a complex protein composed of four subunits. Each subunit consists of a protein, or polypeptide chain, that enfolds a heme group. Each heme contains iron (Fe 2+ ) that can bind a molecule of oxygen. The iron gives blood its red color. After the first year of life, 95-97% of the hemoglobin molecules contain two pairs of polypeptide chains designated alpha and beta. This form of hemoglobin is called hemoglobin A.
Hemoglobin is most commonly measured in whole blood. Hemoglobin measurement is most often performed as part of a complete blood count (CBC), a test that includes counts of the red blood cells, white blood cells, and platelets (thrombocytes).
Some people inherit hemoglobin with an abnormal structure. The abnormal hemoglobin results from a point mutation in one or both genes that code for the alpha or beta polypeptide chains. Examples of hemoglobin abnormalities resulting from a single amino acid substitution in the beta chain are sickle cell and hemoglobin C disease. Most abnormal hemoglobin molecules can be detected by hemoglobin electrophoresis, which separates hemoglobin molecules that have different electrical charges.
No special preparation is required other than cleaning and disinfecting the skin at the puncture site. Blood is collected in a tube by venipuncture. The tube has an anticoagulant in it so that the blood does not clot in the tube, and so that the blood will remain a liquid.
Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn, and lying down and relaxing for awhile is helpful for these people.
Other than potential bruising at the puncture site, and/or dizziness, there are usually no complications associated with this test.
Normal values vary with age and sex, with women generally having lower hemoglobin values than men. Normal results for men range from 13–18 g/dL. For women the normal range is 12–16 g/dL. Critical limits (panic values) for both males and females are below 5.0 g/dL or above 20.0 g/dL.
A low hemoglobin value usually indicates the person has anemia. Different 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 value indicates the body may be making too many red blood cells. Other tests are performed to differentiate the cause of the abnormal hemoblogin level. Laboratory scientists perform hemoglobin tests using automated laboratory equipment. Critically high or low levels should be immediately called to the attention of the patient's doctor.
Chernecky, Cynthia C. and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia: W. B. Saunders Company, 2001.
Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.
Kjeldsberg, Carl R. Practical Diagnosis of Hematologic Disorders. 3rd ed. Chicago: ASCP Press, 2000.
American Association of Blood Banks. 8101 Glenbrook Road, Bethesda, Maryland 20814. (301) 907-6977. Fax: (301) 907-6895. http://www.aabb.org .
Uthman, Ed. Blood Cells and the CBC. 2000 [cited February 17, 2003]. http://web2.iadfw.net/uthman/blood_cells.html .
Victoria E. DeMoranville
Mark A. Best, M.D.