Blood Type Test
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.
- Before a
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