Antibody tests are done to find certain
antibodies that attack red blood cells.
Antibodies are proteins made by the
immune system. Normally, antibodies bind to foreign
substances, such as bacteria and viruses, and cause them to be destroyed.
following conditions cause antibodies to be made.
Human blood is
typed by certain markers (called
antigens) on the surface of
red blood cells. If you get a blood transfusion, the transfused blood must match
your type. That means the transfused blood must have the same antigens as your red blood cells. If
you get a transfusion of blood with antigens different from yours (incompatible
immune system destroys the transfused blood cells. This is called a transfusion
reaction and can cause serious illness or even death. This is why matching
blood type is so important.
Rh is an antigen. The full name
for this antigen is Rhesus factor.
If a pregnant woman with
Rh-negative blood is pregnant with a baby (fetus) with Rh-positive blood,
Rh sensitization may occur. The baby may have
Rh-positive blood if the father has Rh-positive blood. Rh sensitization happens
when the baby's blood mixes with the mother's blood during pregnancy or
delivery. This causes the mother's immune system to make antibodies against the
baby's red blood cells in future pregnancies. This antibody response is called
Rh sensitization and, depending on when it happens, can destroy the red blood
cells of the baby before or after it is born. If sensitization happens, a fetus
or newborn can develop mild to severe problems (called Rh disease or
erythroblastosis fetalis). In rare cases, if Rh
disease is not treated, the fetus or newborn may die.
A woman with
Rh-negative blood can get a shot of
Rh immunoglobulin (such as RhoGAM) that almost always stops sensitization
from occurring. Problems from Rh sensitization have become very rare since Rh
immunoglobulin was developed.
A type of
hemolytic anemia called autoimmune hemolytic anemia is
a rare disease that causes antibodies to be made against a person's own red
Two blood tests can check for antibodies that attack
red blood cells: the direct Coombs test and the indirect Coombs test. The
direct Coombs test is done on a sample of red blood cells from the body. It
detects antibodies that are already attached to red blood cells. The indirect
Coombs test is done on a sample of the liquid part of the blood (serum). It
detects antibodies that are present in the bloodstream and could bind to
certain red blood cells, leading to problems if blood mixing occurs.
The direct Coombs test finds
antibodies attached to your red blood cells. The antibodies may be those your
body made because of disease or those you get in a blood transfusion.
The direct Coombs test also may be done on a newborn baby with
Rh-positive blood whose mother has
Rh-negative blood. The test shows whether the mother
has made antibodies and if the antibodies have moved through the
placenta to her baby.
The indirect Coombs test
finds certain antibodies that are in the liquid part of your blood (serum).
These antibodies can attack red blood cells but are not attached to your red
blood cells. The indirect Coombs test is commonly done to find antibodies in a
recipient's or donor's blood before a transfusion.
A test to
determine whether a woman has Rh-positive or Rh-negative blood (Rh antibody
titer) is done early in pregnancy. If she is Rh-negative, steps can be taken to
protect the baby.
You do not need to do anything before
you have this test.
The health professional drawing blood
The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
There is very little chance of a problem from
having blood sample taken from a vein.
Antibody tests (Coombs tests) are done to find antibodies that attack
red blood cells.
No antibodies are found. This is called a
negative test result.
Reasons you may not be able to
have the test or why the results may not be helpful include:
A newborn baby (whose mother has
Rh-negative blood) may have a direct Coombs test to
check for antibodies against the baby's red blood cells. If the test is
positive, the baby may need a transfusion with compatible blood to prevent
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
December 30, 2011
E. Gregory Thompson, MD - Internal Medicine
& W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
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