Antiglobulin Test
Antiglobulin Test are blood tests that identify the causes of anemia.
The indirect Coombs' test measures the presence of antibodies to red blood cells in the
blood.
The direct Coombs' test measures the presence of antibodies on
the surface of red blood cells.
Purpose of Antiglobulin Test
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 Antiglobulin Test 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 Antiglobulin
Test 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. This disease is called erythroblastosis
fetalis
Many times the cause cannot be identified.
Information about the Antiglobulin Test
There are two Antiglobulin Test. 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 for Antiglobulin Test
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
Antiglobulin Test 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 Antiglobulin Test 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.
Antiglobulin Test 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. |
