The purified protein derivative is an antigen (a substance that stimulates the immune system to eliminate or fight foreign substances in the body), which is injected under the skin in the forearem. After 48 to 72 hours the injection site is evaluated by a physician. This skin test helps determine if a person has ever been infected by the microorganism that causes tuberculosis.
Update Date 11/20/2013
Updated by: Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School; Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.