The PPD (purified protein derivative) test, also known as the tuberculin skin test, is used to determine if someone has been exposed to the TB bacteria. A small amount of tuberculin is injected under the skin of the forearm, and after 48-72 hours, a healthcare provider will check for a reaction, which may indicate TB exposure. A raised bump or swelling at the injection site suggests possible exposure, but further testing is needed to confirm active TB.