The tuberculin skin test (TST), also known as the Mantoux test, is a diagnostic tool used to determine if a person has been exposed to the tuberculosis (TB) bacteria. During the test, a small amount of purified protein derivative (PPD) is injected just under the skin of the forearm. After 48 to 72 hours, a healthcare professional examines the injection site for any swelling or redness. A positive result indicates that the person may have been exposed to TB, but it does not confirm an active infection. Further tests, such as a chest X-ray or sputum test, may be needed to diagnose active TB disease.