Tuberculosis Skin Test
(PPD Skin Test)
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
What is the tuberculosis skin test?
The tuberculosis skin test (also known as the tuberculin test or PPD test) is a test used to determine if someone has developed an immune response to the bacterium that causes tuberculosis (TB). This response can occur if someone currently has TB, if they were exposed to it in the past, or if they received the BCG vaccine against TB (which is not performed in the U.S.).
The tuberculin skin test is based on the fact that infection with M. tuberculosis produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium. The components of the organism are contained in extracts of culture filtrates and are the core elements of the classic tuberculin PPD (also known as purified protein derivative). This PPD material is used for skin testing for tuberculosis. Reaction in the skin to tuberculin PPD begins when specialized immune cells, called T cells, which have been sensitized by prior infection, are recruited by the immune system to the skin site where they release chemical messengers called lymphokines. These lymphokines induce induration (a hard, raised area with clearly defined margins at and around the injection site) through local vasodilation (expansion of the diameter of blood vessels) leading to fluid deposition known as edema, fibrin deposition, and recruitment of other types of inflammatory cells to the area.
An incubation period of two to 12 weeks is usually necessary after exposure to the TB bacteria in order for the PPD test to be positive.
Next: How is the tuberculosis skin test administered? »
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