Antiglobulin Tests Health Article

Definition

Antiglobulin (Coombs') tests are blood tests that identify the causes of immune-mediated anemia or hemolysis. Antiglobulin tests utilize an antibody to human immunoglobulin in order to detect antibody coated (sensitized) cells. In addition to being a medical test that is ordered by a physician, the antiglobulin test is a procedure employed in various blood banking and immunology methods in order to detect immune complex formation. This test is also the basis for some blood typing tests.

Purpose

Antiglobulin tests are used to detect antibodies in serum or attached to cells. The test is ordered as a medical laboratory test to aid in the differential diagnosis of anemia. Anemia refers to blood with abnormally low oxygen-carrying capacity. The hemoglobin in red blood cells carries oxygen. Anemia may be caused by low numbers of red blood cells or a low level of functional hemoglobin. One of the many causes of anemia is destruction of red blood cells, a process called hemolysis (hemo means blood and lysis means disintegration). Any anemia caused by a shortened red blood cell survival in the circulation is called a hemolytic anemia. The causes of hemolytic anemia include septic shock, mechanical trauma, intrinsic red blood cell membrane defects or biochemical (enzyme) defects that make the cells more fragile, and antibody induced destruction. The antiglobulin test identifies red blood cells coated (sensitized) by antibody, and therefore, hemolytic anemia caused by immune mediated destruction. The test also identifies red blood cells coated by complement. Complement is a series of proteins that attach to the cell following the reaction of some types of antibodies with antigens on the cell surface. Complement components undergo changes after attachment to the red cells that damage the membrane and serve as binding sites for phagocytes that remove the red cells from the blood as they pass through the spleen and liver.

Immune-mediated hemolytic anemia can result from a transfusion reaction in which antibodies formed by the recipient attach to and destroy the donor's red blood cells, or it can result from the production of antibodies that attach to the surface of the person's own red cells (autoimmune hemolytic anemia).

Autoimmunity is the cause of many systemic collagen-vascular diseases, including rheumatoid arthritis and systemic lupus erythematosus, and several organ-specific diseases such as type I diabetes mellitus and chronic lymphocytic thyroiditis (hypothyroidism). Some persons with systemic autoimmune disease produce autoantibodies to their red cells, and the antiglobulin test is used to identify these antibodies. In addition, the antiglobulin test may be employed to detect the antibodies responsible for the destruction of the target tissue in autoimmune diseases.

Causes of immune-mediated hemolytic anemia include:

• drugs such as penicillin, methyldopa (lowers blood pressure), and quinidine (treats heart rhythm disturbances)
• cancers of the lymph system, including Hodgkin's disease and lymphomas
• some viral infections
• collagen-vascular diseases
• incompatible blood transfusions
• Rh incompatibility between a mother and fetus (hemolytic disease of the newborn)

In some cases, the cause of an autoimmune hemolytic anemia cannot be identified.