How the test is performed: Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. There is no special preparation necessary for this test. How the test will feel: When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. Risks associated with venipuncture are slight: Excessive bleeding. Fainting or feeling lightheaded. Hematoma (blood accumulating under the skin). Infection (a slight risk any time the skin is broken). Multiple punctures to locate veins. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.