The tuberculosis skin test, also known as the tuberculin test or PPD test, is performed by injecting tuberculin into the skin and determining whether the nodules are negative or positive according to their size, as follows: 1. If the diameter of the nodules is <0.5 cm, it is generally considered positive, suggesting no infection with Mycobacterium tuberculosis or BCG vaccination. 2. If the diameter of the nodule is >0.5 cm and less than 2 cm, it is considered positive and the patient is considered to have been infected with Mycobacterium tuberculosis or vaccinated with BCG. 3. In adults, active tuberculosis is considered a possibility and further examination is needed.