Test Overview
Coombs 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.
The
following conditions cause antibodies to be made.
Transfusion reaction
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
blood), your
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 sensitization
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.
Autoimmune hemolytic anemia
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
blood cells.
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.
Why It Is Done
Direct Coombs test
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.
Indirect Coombs test
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.
How To Prepare
You do not need to do anything before
you have this test.
How It Is Done
The health professional drawing blood
will:
- Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.
- Clean the needle site with
alcohol.
- Put the needle into the vein. If the needle is not placed
correctly or if the vein collapses, more than one needle stick may be
needed.
- Hook a tube to the needle to fill it with
blood.
- Remove the band from your arm when enough blood is
collected.
- Put a gauze pad or cotton ball over the needle site as
the needle is removed.
- Put pressure on the site and then put on a bandage.
How It Feels
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
pinch.
Risks
There is very little chance of a problem from
having blood sample taken from a vein.
- You may get a small bruise at the site. You can
lower the chance of bruising by keeping pressure on the site for several
minutes.
- In rare cases, the vein may become swollen after the blood
sample is taken. This problem is called phlebitis. A warm compress can be used
several times a day to treat this.
Results
Antibody tests (Coombs tests) are done to find antibodies that attack
red blood cells.
Normal
No antibodies are found. This is called a
negative test result.
- Direct Coombs test. A negative test result means
that your blood does not have antibodies attached to your red blood
cells.
- Indirect Coombs test. A negative test result
means that your blood is compatible with the blood you are to receive by
transfusion. A negative indirect Coombs test for Rh factor (Rh antibody titer)
in a pregnant woman means that she has not developed antibodies against the
Rh-positive blood of her baby. This means that
Rh sensitization has not occurred.
Abnormal
- Direct Coombs test. A positive result means your
blood has antibodies that fight against red blood cells. This can be caused by
a transfusion of incompatible blood or may be related to conditions such as
hemolytic anemia
or hemolytic disease of the newborn (HDN).
- Indirect Coombs test. A positive test result
means that your blood is incompatible with the donor's blood and you can't
receive blood from that person. If the Rh antibody titer test is positive in a
woman who is pregnant or is planning to become pregnant, it means that she has
antibodies against Rh-positive blood (Rh sensitization). She will be tested
early in pregnancy to check the blood type of her baby. If the baby has
Rh-positive blood, the mother will be watched closely throughout the pregnancy
to prevent problems to the baby's red blood cells. If sensitization has not
occurred, it can be prevented by a shot of
Rh immunoglobulin.
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include:
- Having a blood transfusion in the
past.
- Being pregnant within
the past 3 months.
- Taking some medicines, such as cephalosporins,
sulfa medicines, tuberculosis medicines, insulin, and tetracyclines.
What To Think About
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
anemia.
References
Other Works Consulted
- Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
- 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.
Credits
ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Adam Husney, MD - Family Medicine
Martin J. Gabica, MD - Family Medicine
Specialist Medical ReviewerMartin J. Gabica, MD - Family Medicine
Current as ofOctober 14, 2016