Your blood will be tested to determine your blood type — classified as A, B, AB or O — and your Rh factor. The Rh factor refers to the presence or absence of a specific antigen — a substance capable of stimulating an immune response — in the blood. You'll be classified as Rh positive or Rh negative, meaning you do or don't carry the antigen. This information is important because your blood type and Rh factor must be compatible with the blood type and Rh factor of the person receiving your blood.

Your blood will also be tested for bloodborne diseases, such as hepatitis, HIV and syphilis. If these tests are negative, the blood is distributed for use in hospitals and clinics. If any of these tests are positive, the donor center notifies you, and your blood is discarded.