Alternative Names cross matching, Rh typing, ABO blood typing
Definition Blood typing is performed to determine a person's blood type. This is 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 antenatal 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. 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 After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding.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.
What is involved in preparation for the test? A person should request specific instructions from his or her doctor. Generally, no special preparation is required.
What do the test results mean? Forward typing means that:
Type A blood clots when mixed with anti-A serum.
Type B blood clots when mixed with anti-B serum.
Type AB blood clots when mixed with both anti-A and anti-B serums.
Type O blood does not clot when mixed with either anti-A or anti-B serum.
Rh-positive blood clots when mixed with anti-Rh serum.
Rh-negative blood does not clot when mixed with anti-Rh serum.
Reverse typing means that:
Serum from type A blood clots when mixed with type B blood.
Serum from type B cells clots when mixed with type A blood.
Serum from type O blood clots when mixed with type A and type B blood.
Serum from type AB blood does not clot when mixed with type A or type B blood.
What this means for transfusions:
Persons with type A blood can receive blood transfusions from donors with type A or type O blood.
Persons with type B blood can receive transfusions from donors with type B or type O blood.
Persons with type AB blood can receive transfusions from donors with type AB, type A, type B, or type O blood.
Persons with type O blood can receive transfusions from persons with type O blood.
Persons with Rh-positive blood can receive transfusions from donors with Rh-positive and Rh-negative blood.
Persons with Rh-negative blood can receive transfusions from donors with Rh-negative blood.
Author: Reviewer: HealthAnswers Australia Medical Review Panel Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia Last Updated: 1/10/2001 Contributors Potential conflict of interest information for reviewers available on request
