The tuberculosis skin test is also known as the tuberculin test or PPD test. The PPD test is used to determine if someone has developed an immune response to the bacterium that causes tuberculosis (TB). The standard recommended tuberculin test is the Mantoux test, which is administered by injecting a 0.1 mL of liquid containing 5 TU (tuberculin units) PPD (purified protein derivative) into the top layers of skin of the forearm. Skin tests should be read 48-72 hours after the injection. The basis of the reading of the skin test is the presence or absence and the amount of induration (localized swelling. A negative test does not always mean that a person is free of tuberculosis. A person who received a BCG vaccine (administered in some countries but not the U.S.) against tuberculosis may also have a positive skin reaction to the TB test. This is also offered as a blood test.