PPD skin test is one of the main means to determine whether the organism is infected with Mycobacterium tuberculosis. According to the newly revised national Diagnostic Criteria for Tuberculosis in 2017, if the average diameter of hard nodes at the injection site exceeds 5mm, it is a positive reaction, where if the injection site reacts more strongly with blisters, necrosis, ulcers, double circles, lymphangitis, etc., or the average diameter of hard nodes exceeds 15mm or more, it is called a strong positive reaction of PPD.The contents of a positive PPD test imply the following specifically. A positive tuberculin skin test alone indicates that the organism has been infected by Mycobacterium tuberculosis. If there is no abnormality or no tuberculosis lesion by chest X-ray examination and no clinical evidence of tuberculosis in other parts of the body, it can be considered as a person infected with Mycobacterium tuberculosis; 2. A positive tuberculin skin test, if there are suspicious symptoms of tuberculosis, abnormal chest X-ray examination findings or highly suspected tuberculosis lesion, negative bacteriological examination of tuberculosis 3, young children under 3 years of age who have not been vaccinated with BCG and have a positive or strong positive reaction to the tuberculin test, have a history of close family contact with tuberculosis and are accompanied by fever, wasting and other uncomfortable symptoms, should indicate that there are active foci of tuberculosis in the body; 4, newborns or young children who have been vaccinated with BCG for 4-8 weeks have a positive tuberculin skin test, indicating that BCG vaccination has been successful and The artificial immunity of the organism has been established, but the intensity of the reaction of the skin test for tuberculin after BCG vaccination is significantly weaker than that of the natural infection of Mycobacterium tuberculosis.