A blood sample is needed and will be drawn from a vein.
The test to determine your blood group is called ABO typing. Your blood sample is mixed with antibodies against type A and B blood, and the sample is checked to see whether or not the blood cells stick together (agglutinate). If blood cells stick together, it means the blood reacted with one of the antibodies.
The second step is called back typing. The liquid part of your blood without cells (serum) is mixed with blood that is known to be type A and type B. Persons with type A blood have anti-B antibodies, and those with type B blood have anti-A antibodies. Type O blood contains both types of antibodies. These two steps can accurately determine your blood type.
Blood typing is also done to tell whether or not you have a substance called Rh factor on the surface of your red blood cells. If you have this substance, you are considered Rh+ (positive). Those without it are considered Rh- (negative). Rh typing uses a method similar to ABO typing.