Determining blood type requires that both ABO typing and a test for the blood's Rh factor are performed by a trained professional, states MedlinePlus. The test has several steps that use the reaction of blood to sets of antibodies to differentiate between blood types.
ABO typing requires mixing a blood sample with serums of antibodies for blood types A and B, states MedlinePlus. The sample is examined for a reaction in which the blood sticks together. If blood cells stick together when mixed with anti-A serum, the sample is an A blood type. Type B blood reacts to the anti-B serum. If the sample reacts to both serums, the type is AB. If it reacts with neither serum, then the blood is type O. Rh typing is performed using anti-Rh serum, with a reaction meaning the sample is Rh-positive.