A screening test for tuberculosis (TB). Purified protein derivative (PPD) extracted from the bacterium that causes tuberculosis is injected just below the skin (intradermally). After 48 to 72 hours, a health care professional checks the site of injection for a reaction that indicates that the person has been exposed to TB. Following a positive TB skin test, additional tests are necessary to determine whether a person actually has active TB (TB disease). Certain populations, such as children, the elderly, or people with weakened immune systems, may have smaller, delayed, or negative reactions to the TB test even if they are infected with TB.