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Blood type tests are done before a person gets
a blood transfusion and to check a pregnant woman's blood type. Human blood is
typed by certain markers (called
antigens) on the surface of red blood cells. Blood
type may also be done to see if two people are likely to be blood
The most important antigens are blood group antigens
(ABO) and the Rh antigen, which is either present (positive, +) or absent (negative, -). So the two most common blood type tests are
the ABO and Rh tests.
The ABO test shows that people have one
of four blood types: A, B, AB, or O. If your red blood cells have:
Blood received in a transfusion must have the same
antigens as yours (compatible blood). If you get a transfusion that has
different antigens (incompatible blood), the antibodies in your plasma will
destroy the donor blood cells. This is called a transfusion reaction, and it
occurs immediately when incompatible blood is transfused. A transfusion
reaction can be mild or cause a serious illness and even death.
Type O-negative blood does not have any antigens. It is called the
"universal donor" type because it is compatible with any blood type. Type
AB-positive blood is called the "universal recipient" type because a person who
has it can receive blood of any type. Although "universal donor" and "universal
recipient" types may be used to classify blood in an emergency, blood type
tests are always done to prevent transfusion reactions.
antigens (other than A, B, and Rh) that occur on red blood cells can sometimes
also cause problems and so are also checked for a match before giving a blood
Serious transfusion reactions are rare today because
of blood type tests.
Rh blood type checks for the Rh antigen
(also called the Rh factor) on red blood cells. If your red blood cells:
For example, if you have the A and Rh antigens, your
blood type is A-positive (A+). If your blood has the B antigen but not the Rh
antigen, your blood type is B-negative (B–).
Rh blood type is
especially important for pregnant women. A problem can occur when a woman who
has Rh-negative blood becomes pregnant with a baby (fetus) that has
Rh-positive blood. This is called Rh incompatibility. If the blood of an
Rh-positive baby mixes with the blood of an Rh-negative mother during pregnancy
or delivery, the mother's immune system makes antibodies. This antibody
response is called
Rh sensitization and, depending on when it occurs, can
destroy the baby's red blood cells.
Rh sensitization does not
generally affect the health of the baby during the pregnancy in which the
sensitization occurs. But the health of a baby with Rh-positive blood during a
future pregnancy is more likely to be affected. After sensitization has
occurred, the baby can develop mild to severe problems (called Rh disease or
erythroblastosis fetalis). In rare cases, if Rh
disease is not treated, the baby may die.
An Rh test is done in
early pregnancy to check a woman's blood type. If she is Rh-negative, she can
get a shot of
Rh immunoglobulin that almost always prevents sensitization from occurring.
Problems from Rh sensitization have become very rare since Rh immunoglobulin
A blood type test is done:
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 a blood sample taken from a vein.
Blood type tests are done before a person
gets a blood transfusion and to check a pregnant woman's blood type. The
following table shows the compatibility of blood types between blood donors and
Read the table as follows: A person who has A-negative
blood can receive A-negative or O-negative blood.
A-, O- blood
A-, A+, O-, O+ blood
B-, O- blood
B-, B+, O-, O+ blood
AB-, O- blood
AB-, AB+, A-, A+, B-, B+, O-, O+
O-, O+ blood
antigens (other than A, B, and Rh) on the red blood
cells are also checked for a match before a blood transfusion.
Reasons you may not be able to
have the test or why the results may not be helpful include:
Stanford University School of Medicine (2011). Blood types in the U.S. Available online: http://bloodcenter.stanford.edu/about_blood/blood_types.html.
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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