Blood typing is performed to determine a person's blood type. This determined by the type of antigens or markers that are on the surface of red blood cells (either "A" or "B") and if there are antibodies to a portion of the blood type known as the Rh factor (either "positive" or "negative").

In the case of a transfusion, a person's blood type needs to be compatible with the donor's blood type or an allergic-type reaction can occur. It is also important to know the Rh factor status when a man and a woman are having a baby. If a pregnant woman is Rh negative and her baby is Rh positive, the mother's immune system can sometimes attack the baby's blood cells because of the differing blood types. All pregnant women should have a blood test to see if they are at risk. This is done as part of routine prenatal care. An injection of Rho D Immune Globulin can be given during pregnancy to prevent this reaction, which may be harmful to the baby.

Who is a candidate for the test?

All pregnant woman and all those who need a transfusion need this test. All donated blood has this blood type test performed on it so that donor and recipient blood types match. Paternity testing often involves blood typing, which may or may not be able to prove if someone could be the parent of a child. There are other, rarer indications for blood typing, such as genetic studies.

How is the test performed?

A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial for testing in the laboratory. After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding.

Both Rh typing and ABO typing are done on the blood sample in the laboratory to determine the blood type.

Rh typing: The blood is mixed with serum containing anti-Rh antibodies. If the blood clots, then the blood contains Rh antigens, and it is known as Rh-positive. If the blood does not clot, it does not contain Rh antigens, and it is known as Rh-negative.

ABO typing: The ABO typing process consists of forward and reverse typing.

In forward typing a sample of the blood is mixed with serum that contains antibodies against type A blood ("anti-A serum"). Another sample of blood is then mixed with serum that contains antibodies against type B blood ("anti-B serum"). Patterns of clotting are observed and recorded.

In reverse typing a sample of blood is mixed with type A and type B blood, and clotting patterns are observed and recorded.