To perform this test, 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 rubber tube is removed.
Blood flows from the vein through the needle, and is collected in a syringe or vial.
After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding. Next, the blood sample is taken to the laboratory. In the laboratory, a test solution is combined with the blood sample. The test solution contains either an antibody or antigen coupled to an enzyme. For example, if the doctor wants to test for an antibody, the test solution will contain an antigen that will react with that antibody.