Test Overview

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
relatives.

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.

ABO test

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.

Minor
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
transfusion.

Serious transfusion reactions are rare today because
of blood type tests.

Rh test

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
    Rh-positive.
  • Do not have the Rh antigen, your blood is
    Rh-negative.

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
was developed.

Why It Is Done

A blood type test is done:

  • Before a person gets a blood
    transfusion.
  • Before a person donates blood.
  • Before a
    person donates an organ for transplantation.
  • Before
    surgery.
  • When a woman is planning to become pregnant or first
    becomes pregnant.
  • To show whether two people could be blood
    relatives.
  • 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
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 to the site and then 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 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
    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

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
recipients.

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: Can receive:

A- blood

A-, O- blood

A+ blood

A-, A+, O-, O+ blood

B- blood

B-, O- blood

B+ blood

B-, B+, O-, O+ blood

AB- blood

AB-, O- blood

AB+ blood

AB-, AB+, A-, A+, B-, B+, O-, O+
blood

O- blood

O- blood

O+ blood

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
    transplant.

What To Think About

  • Severe transfusion reactions are rare today
    because blood type is always done before a person receives a blood
    transfusion.
  • 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).

References

Citations

  1. 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 (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 Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine

Current as ofOctober 9, 2017