The Mycobacterium tuberculosis skin test, also known as the PPD test, is also known as the tuberculin test, also known as the tuberculosis test. When it appears positive, it mostly indicates that you have been infected with Mycobacterium tuberculosis, and a negative test indicates that you have not been infected with Mycobacterium tuberculosis, so a positive test usually does not turn negative. If the test is negative, a negative test can be changed to a positive one by preventing tuberculosis with BCG vaccination. If it is still negative after BCG injection, it usually means that the immunization has failed and should be reinjected. A positive skin test for Mycobacterium tuberculosis can only indicate an infection with Mycobacterium tuberculosis and cannot determine whether there is active tuberculosis, because it is not certain that the body will develop the disease when infected with Mycobacterium tuberculosis. Whether the disease develops depends on the immunity of the human body, as well as the number and virulence of the infected Mycobacterium tuberculosis. The higher the number, the stronger the virulence, the lower the immunity, the more likely the disease will develop, and vice versa, the incidence decreases.