The results of the PPD test are mainly based on the size of the hard nodules. It is divided into three kinds of results: negative, positive and strong positive. If it is negative, it indicates that there is no infection with Mycobacterium tuberculosis and no BCG vaccination, and such people are recommended to get BCG vaccination. If the result is positive, it indicates that the person has been infected with Mycobacterium tuberculosis or vaccinated with BCG, but it is impossible to tell which one is the cause of the infection and needs to be analyzed together with other test results. If it is strongly positive, active tuberculosis is considered for children, and if adults it is considered possible active tuberculosis, which also needs to be combined with other tests.