The Mantoux tuberculin skin test is a diagnostic tool used to determine if a person has been exposed to the tuberculosis (TB) bacteria. A small amount of purified protein derivative (PPD) is injected just under the skin of the forearm. After 48 to 72 hours, the injection site is examined for a reaction, typically swelling or redness, which indicates potential exposure to TB. A positive result does not confirm active TB but suggests that the person has been infected at some point. Further tests, such as a chest X-ray or sputum test, may be needed to diagnose active TB disease.