Tuberculosis PPD, also known as tuberculin test, can suggest some clinical significance by intra-dermal injection of a certain concentration of tuberculin from Mycobacterium tuberculosis and observation of the condition and performance of the skin site after the injection. These clinical implications are mainly for the basis of BCG vaccination, the effect after vaccination, the diagnosis of tuberculosis, and the evaluation of the efficacy of treatment of tuberculosis. The skin at the site of the skin test is negative if there is no erythema, nodules, or any other clinical manifestations. If the skin at the skin test site shows redness and hard nodules of more than 0.5 cm in diameter, the test is positive.