What are Blood Typing and Crossmatching?
Blood typing is a test used to find out what specific blood
type you have. Blood types are categorized using a blood typing system called
the ABO system. The four main categories
in the ABO system are:
- Type A
- Type B
- Type AB
- Type O
Your blood type depends on the presence of certain antigens on your red blood cells
(RBCs). Antigens are proteins that cause your immune system to produce
antibodies. They can be foreign substances, like a virus or pollen, or something
produced by your body.
If you need a blood transfusion or transplant, another
technique called crossmatching is
performed prior to the transfusion or transplant. There are several other
antigens beside the major ones listed above. Crossmatching will help to detect
the minor ones. Blood typing and crossmatching together are used to prevent
harmful interactions between your blood and donor blood.
What are Blood Typing and Crossmatching Used For?
Doctors and healthcare workers use blood typing and crossmatching
to make sure that the patient receiving a blood transfusion or transplant is
getting the correct type of blood. If you receive a blood transfusion and the
wrong blood type is used, your immune system may attack the transfused blood. People
who are receiving a procedure or surgery where significant blood loss could
occur should also receive blood typing and crossmatching prior to the
Healthcare workers use the following guidelines:
- People with type A blood should only receive
types A and O
- People with type B blood should only receive
types B and O
- People with type AB blood may receive A, B, AB,
and O (this is why blood type AB is referred to as the “universal recipient”)
- People with type O blood should only receive
type O blood
Anyone can receive type O blood. Someone with blood type O
is called a “universal donor.” This is why type O blood is often used in
emergencies when there is not enough time to perform a blood typing test (NIH).
Crossmatching is used by a doctor to make sure that the
specific donor blood that will be used during a transfusion does not react with
a patient’s blood. It is basically a transfusion done in a test tube. The
process takes 45 minutes to an hour and should be done at least three days
prior to the transfusion to be accurate (University
The tests are also used on pregnant women in order to
prevent a type of anemia called hemolytic
disease developing in a newborn (HDM). Babies who have a blood type that is
different from their mother are at risk of developing this condition.
How are Blood Typing and Crossmatching Performed?
Blood typing and crossmatching are performed by a trained
health care practitioner at a doctor’s office or blood bank. Blood is drawn
from a vein. The healthcare practitioner will typically use a vein located
somewhere on the inside of the elbow. A blood draw usually includes the
- The puncture site is disinfected
with an antiseptic
- A health care provider wraps
an elastic band around the upper part of the arm in order to make the vein
swell up with blood
- A needle is inserted
gently into the chosen vein to collect blood into a tube
- The band is removed
from the arm
- The blood sample is sent
to a blood bank laboratory for analysis by trained technologists
In the laboratory, the sample of blood is mixed with commercially-prepared
antibodies against type A and B blood. If the blood cells agglutinate (stick together) it means that the blood has had a
reaction with one of the antibodies. Another step, called back typing, is performed next. The blood serum is stirred together
with type A and type B blood.
Blood typing also determines whether a patient has proteins called
Rh factor on their RBCs. People with
Rh factor are designated Rh positive (Rh+), while people without Rh factor are
called Rh negative (Rh-). Your Rh type is also used to decide which type of
blood you can safely receive during a transfusion.
How Should You Prepare for the Test?
There is no specific preparation
required for this test.
What are the Risks of Blood Typing and Crossmatching?
There is a risk of bruising,
bleeding, or infection at the puncture site. Some pain or a sharp prick may be
felt when the needle is inserted.
What do Blood Typing and Crossmatching Results Mean?
There is no such thing as a “normal” blood type. Blood type
is inherited, meaning you are born with a certain blood type, just like you are
born with a certain eye color. O positive is the most common of all the blood
Results are determined as follows:
- If your blood cells agglutinate when mixed with anti-A
serum, you have blood type A
- If they agglutinate when mixed with anti-B serum, you have
blood type B
- If they agglutinate when mixed with both serums, you have
blood type AB
- If they do not agglutinate when either serum is added, you
have blood type O
People with type A blood will have anti-B
antibodies. People with type B blood will have anti-A antibodies. People with
type O blood will have both. Therefore:
- If your blood clumps
only when the B cells are added, you have blood type A
- If your blood clumps
only when the A cells are added, you have blood type B
- If your blood clumps in both cases, you have type O
- If your blood does not clump
when both types of blood are added, you have blood type AB
- If your blood sticks together when
anti-Rh serum is added, you are Rh+
- If your blood does not clump when
anti-Rh serum is added, you are Rh-