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:
The A antigen, you have type A blood. The
liquid portion of your blood (plasma) has antibodies that attack type B blood. About 42% of people (42 in 100) in the United States have type A blood, with 6% having A-negative (A-) blood and 36% having A-positive (A+) blood.footnote 1
The B antigen,
you have type B blood. Your plasma has antibodies that attack type A
blood. About 10% of people (10 in 100) in the U.S. have type B blood, with 2% having B-negative (B-) blood and 8% having B-positive (B+) blood.footnote 1
Neither the A nor B antigen, you have type O blood. Your plasma
has antibodies that attack both type A and type B blood. About 44% of people (44 in 100) in the U.S. have type O blood, with 7% having O-negative (O-) blood and 37% having O-positive (O+) blood.footnote 1
Both the A and B
antigens, you have type AB blood. Your plasma does not have antibodies against
type A or type B blood. About 4% of people (4 in 100) in the U.S. have type AB blood, with 1% having AB-negative (AB-) blood and 3% having AB-positive (AB+) blood.footnote 1
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:
Have the Rh antigen, your blood is
Do not have the Rh antigen, your blood is
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
Why It Is Done
A blood type test is done:
Before a person gets a blood
Before a person donates blood.
person donates an organ for transplantation.
When a woman is planning to become pregnant or first
To show whether two people could be blood
To check the identify of a person suspected of
committing a crime.
How To Prepare
You do not need to do anything before
you have this test.
How It Is Done
The health professional drawing blood
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
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
Hook a tube to the needle to fill it with
Remove the band from your arm when enough blood is
Put a gauze pad or cotton ball over the needle site as
the needle is removed.
Put pressure to the site and then a
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
There is very little chance of a problem from
having a 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
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.
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.
Blood types that match
A person who has:
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
Minor antigens (other than A, B, and Rh) on the red blood
cells are also checked for a match before a blood transfusion.
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 3 months before the blood type test.
Having a bone marrow
What To Think About
Severe transfusion reactions are rare today
because blood type is always done before a person receives a blood
Blood type may be done to check the identity of birth
parents. For example, in a paternity case, if the blood types of a mother and
her child are checked against blood types of the possible fathers, the real
father can sometimes be found by the blood type match. But blood type is more
useful in proving that a man is not the father than it is in proving that he is
the father. Other tests, such as a human leukocyte antigen (HLA) type, may be
done. To learn more, see the topic Human Leukocyte Antigen (HLA).