What is the Coombs test?
Coombs test is used to detect antibodies that act on the surface of red blood cells. The presence of these antibodies indicates a condition called hemolytic anemia. In this condition, the blood does not have enough red blood cells because the red blood cells are destroyed prematurely.
Healthy red blood cells survive for about 120 days. In people with hemolytic anemia, red blood cells are destroyed well before the 120-day marker. The bone marrow may be able to compensate for prematurely destroyed red blood cells by working extra hard to produce more red blood cells. However, this extra effort may not be enough to combat hemolytic anemia.
Hemolytic anemia can be hereditary or acquired. Acquired autoimmune hemolytic anemia (AIHA), found in approximately 10% of lupus patients, is caused by the immune system attacking red blood cells.
People with AIHA may experience weakness, dizziness, fever, and jaundice. Treatment requires high-dose steroids such as prednisone and may be supplemented with immunosuppressive drugs such as azathioprine (Imuran).
What are the types of Coombs tests?
There are two types of Coombs test.
Direct tests look for antibodies attached to red blood cells. Indirect tests look for antibodies floating in the liquid part of the blood called serum.
Why take the indirect Coombs test?
Doctors use the indirect Coombs test, also called the IAT, to rule out problems.
A blood test is done before the transfusion to make sure that the donated blood does not contain antibodies that cause adverse reactions. This is part of the “type and page” process.
Pregnant women undergo prenatal antibody screening with the indirect Coombs test. The mother’s blood is tested for the presence of antibodies that can infect and harm the fetus.
Why take the direct Coombs test?
A direct Coombs test (DAT) may help explain why you don’t feel well or have symptoms that indicate a blood-related problem.
If the donor’s blood is not well matched, he may become sick after the transfusion. Your body may recognize those other blood cells as foreign and make antibodies to get rid of them.
A blood disorder called autoimmune hemolytic anemia occurs when antibodies destroy your red blood cells faster than your body can make them. You can get it for the following reasons:
- Diseases such as lupus and leukemia
- Infectious diseases such as mononucleosis
- Medicines containing penicillin
Babies with yellow skin and eyes may have a hemolytic disease of the newborn (HDN). Some maternal antibodies may attack red blood cells. This often happens when some of the baby’s blood types inherited from the father do not mix with the mother’s blood type.
What is the normal result of the Coombs test?
A natural result is called a negative result. This means no cell aggregation and no antibodies against red blood cells.
The normal range may vary slightly between laboratories. Some laboratories use different measurements or test different samples. Consult your provider for the implications of specific test results.
What does an abnormal result in the Coombs test mean?
An abnormal (positive) direct Coombs test means you have antibodies that work against red blood cells. This may be due to:
- Autoimmune hemolytic anemia
- Chronic lymphocytic leukemia or similar disease
- A newborn blood disorder called erythroblastosis fetalis (also called hemolytic disease of the newborn)
- Infectious mononucleosis
- Mycoplasma infection
- Syphilis
- Systemic lupus erythematosus
- Blood transfusion reactions, such as those with inappropriate blood units
Test results may be abnormal for no apparent reason, especially in the elderly.
An abnormal (positive) indirect Coombs test means you have antibodies that work against red blood cells that your body sees as foreign. This can indicate:
- Fetal erythroblastosis
- Matching incompatible blood (for use in blood banks)
What are the risks of the Coombs test?
There are few risks associated with blood sampling. The size of veins and arteries is different in different people and in different parts of the body. Taking blood from some people can be more difficult than others.
Other risks associated with blood sampling are minor, but include:
- Fainting or lightheadedness
- Multiple holes to locate veins
- Hematoma (collection of blood under the skin)
- Excessive bleeding
- Infection (there is a small risk every time the skin is broken)
