Tuberculin test, also known as PPD test, is an intradermal test that diagnoses type IV hypersensitivity reaction caused by Mycobacterium tuberculosis infection by intradermal injection of tuberculin and according to the skin condition of the injection site. Its clinical significance is as follows: 1. For diagnosis and differential diagnosis The tuberculin test has an important role in the diagnosis and differential diagnosis of tuberculosis in adolescents, children and the elderly, and is a more important auxiliary examination method. 2. To provide a basis for BCG vaccination If the tuberculin test is positive, it indicates that the body has been infected with tuberculosis bacteria, and BCG vaccination is no longer necessary. If the tuberculin test is negative, the person is the target of BCG vaccination. The tuberculin test is usually done 3 months after BCG vaccination to find out whether the body has developed immunity to BCG. If the tuberculin test is positive, it indicates that BCG vaccination is successful; otherwise, BCG vaccination should be repeated. Analysis of the results of tuberculin test: 1, no redness and hard nodes at the test site after vaccination (-) 2, red spots or slightly red and swollen at the test site with a diameter of <0.5cm (±); 3, redness and hard nodes at the test site with a diameter of 0.5-0.9cm (+) 4, redness and hard nodes at the test site with a diameter of 1.0-1.9cm (++) 5, redness and hard nodes at the test site with a diameter of ≥2cm (++) diameter ≥ 2cm for (++) 6, in addition to the presence of redness and hard nodes, local blistering and necrosis for (++++).