What is being tested?
Red blood cells (RBCs) have markers or antigens on the surface of the cells. The two major antigens or surface identifiers on human RBCs are the A and B antigens. Your blood is grouped according to the presence or absence of these antigens. People whose red blood cells have A antigens are considered to be blood group A; those with B antigens are group B; those with both A and B antigens are group AB; and those who do not have either of these markers are considered to have blood group O. Another important surface antigen is called Rh factor. If it is present on your red blood cells, your blood type is Rh+ (positive); if it is absent, your blood is type Rh- (negative).

Our bodies naturally produce antibodies against the A and B antigens we do not have on our red blood cells. For example, a person who is blood type A will have antibodies directed against the B surface antigens on red blood cells and someone who is type B will have anti-A antibodies and so on. The following table indicates the type of antibodies a person is expected to have based on their blood type.

A person with bloodtype ...	Will have antibodies to ...
A	B antigen
B	A antigen
AB	Neither antigen
O	A and B antigens

These antibodies are useful for determining a person’s ABO group and are significant in defining the types of blood that they can safely receive.

How is the sample collected for testing?
A sample of blood is drawn from a vein in your arm or from the tip of your finger (fingerstick). In newborns, blood from the umbilical cord or a small amount of blood from a heelstick may be used for testing.

Is any test preparation needed to ensure the quality of the sample?
No special preparation is needed for this test.