Source: U.S. Centers for Disease Control and Prevention (CDC). http://www.cdc.gov

During a Mantoux (PPD) tuberculin skin test, TB antigens are injected under the top layer of skin. A positive reaction, usually a firm red bump where the needle went into your skin, means you were exposed to TB bacteria. This test does not reveal whether you have active TB, which can be spread to others. A trained health professional must interpret your test.