Coombs' Tests

Definition

Coombs' tests are blood tests that identify the causes of anemia.

Purpose

Anemia, which literally means no blood, refers to blood with abnormally low oxygen-carrying capacity. The hemoglobin in red blood cells carries oxygen. One of the many causes of anemia is destruction of red blood cells, a process called hemolysis (hemo means blood and lysis means disintegration). A simple blood count detects anemia. Even the test done before a blood donation can identify anemia. To detect hemolysis requires other tests. The Coombs' tests are conducted in order to determine the cause of anemia.

One characteristic of hemolysis is the autoimmune response against the body's red blood cells. Instead of protecting the body from outside agents, the immune system attacks parts of its own body with a deluge of antibodies. Autoimmunity is thought to be the cause of many collagen-vascular diseases, including rheumatoid arthritis and systemic lupus erythematosus. It is also the cause of the autoimmune hemolytic anemias. The Coombs' tests detect the antibodies responsible for the destruction of the red blood cells.

Causes of autoimmune hemolytic anemia include:

• drugs such as penicillin, methyldopa (lowers blood pressure), and quinidine (treats heart rhythm disturbances)
• cancers of the lymph system—Hodgkin's disease and lymphomas
• virus infections
• collagen-vascular diseases
• mismatched blood transfusions
• Rh incompatibility between a mother and fetus. (erythroblastosis fetalis)

Many times the cause cannot be identified.

Description

There are two Coombs' tests. A direct Coombs' test detects the two different antigens that might induce hemolysis in the patient's red blood cells. An indirect Coombs' test looks for antibodies to someone else's red blood cells in the patient's serum (the blood without the cells). Combining the two tests gives clues to the origin of the hemolysis.

Preparation

No preparation is needed for this test. It will probably be among the second or third set of blood tests done after anemia is diagnosed and there is a suspicion that its cause is hemolysis.

Aftercare

Coombs' tests are done on blood that is drawn from the arm.

Risks

Taking blood for testing is the most common medical procedure performed. The worst complication is a bruise at the site of the puncture or punctures. It is extremely rare for the needle to injure an important structure such as an artery or a nerve.

Normal results

If the Coombs' tests are negative, the anemia is unlikely to be autoimmune, and the hematologist will have to search elsewhere for a cause.

Abnormal results

If the test is positive, the antigens that react will narrow the search for a cause. Coombs' tests are also done for blood transfusion reactions to determine why the transfused blood did not match, and when there is a chance a newborn may have an Rh problem.

Resources

BOOKS

Rosse, Wendell, and H. Franklin Bunn. "Hemolytic Anemias and Acute Blood Loss." In Harrison's Principles of Internal Medicine, edited by Anthony S. Fauci, et al. New York: McGraw-Hill, 1997.

KEY TERMS

Antibody— A protein made by the immune system and used as a weapon against foreign invaders in the body.

Antigen— The chemical that stimulates an immune response.

Anemia— Reduced oxygen-carrying capacity of the blood, due to too little hemoglobin or too few red blood cells.

Collagen-vascular disease— Various diseases inflaming and destroying connective tissue.

Hematologist— Physician who specializes in diseases of the blood.

Hemoglobin— The red pigment in blood that carries oxygen.

Hemolysis— Breaking apart red blood cells.

Rh— A blood typing group, like the ABO system. When a mother is Rh negative and her baby is Rh positive, she may develop antibodies to the baby's blood that will cause it to hemolyze.