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.
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 procedure.
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 of Rochester).
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.
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 following steps:
- 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.
There is no specific preparation required for this test.
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 types (American Red Cross).
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-