A blood type lab test is a simple and routine blood test used to determine your blood type. These tests are commonly performed before an individual gets a blood transfusion, or to check a pregnant woman’s blood type.

Blood type is determined by certain protein markers - known as antigens - on red blood cells. The presence of the Rh antigen specifically, determines whether or not you have a positive or negative blood type.

There are four primary types of blood. These are:

- Type A: The A antigen indicates type A blood. This means the plasma in your blood will attack type B blood. About 36% of people (36 in 100) in the United States have type A blood, with 6% having A-negative (A-) blood and 30% having A-positive (A+) blood.

- Type B: The B antigen indicates type B blood, which means that the plasma in your blood will attack type A blood. About 11% of people (11 in 100) in the U.S. have type B blood, with 2% having B-negative (B-) blood and 9% having B-positive (B+) blood.

- Type O: Type O blood contains neither the A nor B antigen, meaning the plasma in your blood will attack both A and 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.

- Type AB: This blood type contains both A and B antigens, which means your blood does not contain antibodies for either protein. About 48% of people (48 in 100) in the U.S. have type O blood, with 9% having O-negative (O-) blood and 39% having O-positive (O+) blood.

Type O negative blood does not contain any antigens. This is why O negative blood is known as the “universal donor” blood type, as the plasma in O negative blood will not attack any other blood type. Inversely, type AB positive blood is known as the “universal recipient” type.

Blood transfusions require the blood being given to be the same type as that of the recipient. So, if an individual with type B negative blood requires a blood transfusion, they must be given either O negative blood or type B negative blood. If a different blood type is transfused into the body, the plasma of the recipient’s blood will attack the new blood. This leads to a condition known as a transfusion reaction - which can lead to serious illness or death.

Incompatible blood transfusions are rare, due to the accuracy of modern blood type tests.

A blood type test requires a blood sample. These tests can be performed at a primary care facility, and at some minute-clinics. To draw blood, your doctor will either prick your finger or insert a small, thin needle into the forearm. Before blood is drawn, the doctor will clean the area being punctured. You will feel a slight pinch or sting as your finger is pricked or the needle is inserted into your forearm. This is usually a mild sensation and will go away within a few seconds.

As the blood is drawn, it is deposited into a small vial or tube. After the desired amount of blood is taken from the puncture site, the tube will be closed and given to a lab for testing. The puncture site will then be cleaned and bandaged.

You will be able to go about your daily activities as normal after your blood type test. No sedation or preparation is needed for a blood type test, so in most cases, you are free to go once the sample has been collected.

Once the vial or tube containing your blood is given to the laboratory, blood containing the A and B antibodies will be mixed with it. Blood cells that stick together after exposure to an antibody is reacting to the foreign blood type.

Your blood may also be checked for the Rh antigen. If your blood contains this protein, you have a positive blood type. Conversely, the lack of the Rh antigen indicates that you have a negative blood type.

Blood type tests are routine, and your results should be forthcoming shortly after your appointment. In most cases, this may take only a day or two.

Blood type tests rarely require a follow-up appointment. If you are pregnant and your doctor determines that you have a different Rh content than your developing baby, you may be asked to schedule a follow-up appointment. This condition is known as Rh incompatibility and can cause serious adverse reactions.

Rh incompatibility occurs when a mother has negative Rh content in her blood and her child has Rh-positive blood; for instance, the mother has A-negative blood, but the baby has A positive blood. If this blood mixes, the mother’s immune system will produce antibodies to combat the foreign blood (a process known as Rh sensitization). This does not cause problems during the first pregnancy, but if a mother has another child with Rh-positive blood, her plasma will begin to destroy the child’s red blood cells. If left untreated, this condition can lead to serious illness and the death of the unborn child.

If it is determined that a woman is Rh-negative before her pregnancy, she may have an Rh immunoglobulin shot. This prevents the symptoms and complications of Rh sensitization.