A positive PPD test has a certain reference significance for clinical diagnosis of tuberculosis. The test is performed by intradermal injection of tuberculin, and the infection of Mycobacterium tuberculosis is judged according to the skin condition of the injection site after 72 hours. According to the different conditions of the skin at the injection site, the test result can be classified as negative, positive or strong positive: 1. If the skin does not appear hard nodules, only local mild redness or the average diameter of hard nodules is less than 5mm, the result is judged as negative, indicating no tuberculosis infection, early infection has not yet produced metaplasia or the body has lost the ability to respond in severe tuberculosis; 2. If the average diameter of skin nodules is 5 to 19 mm, the result is positive, indicating that the body has a metabolic reaction to Mycobacterium tuberculosis, as seen in patients who have been infected with Mycobacterium tuberculosis in the past and those who have received BCG vaccination, which does not mean that they are suffering from tuberculosis; 3, if the average diameter of skin nodules is >20 mm or there are local blisters and necrosis, the result is strongly positive, indicating that the body may be infected with Mycobacterium tuberculosis and is active, further examination should be performed at this time Further tests such as sputum culture, sputum for antacid bacilli, and bronchoscopic biopsy should be performed to determine whether the patient has tuberculosis. The result of PPD test has some reference significance, but it should not be relied on alone to diagnose tuberculosis, but should be based on the patient’s specific condition and the improvement of related tests to clarify the diagnosis.