A positive TB skin test result indicates infection with Mycobacterium tuberculosis, in which case no further vaccination is required. If the test is negative, the vaccine can be given to prevent infection with Mycobacterium tuberculosis. If it is strongly positive, it is associated with active tuberculosis. The TB skin test, also called the tuberculin test, also known as the PPD test, involves injecting tuberculin toxin into the skin and observing changes in the skin 48-72 hours later. The skin test is negative if the skin appears red, no nodules are present, or if the nodules are small, <0.5 cm in diameter. If the diameter of the nodule is >0.5 cm and less than 2 cm, it is positive. If the diameter of the nodule is >2cm, it is strongly positive. The diagnosis of strong positive is related to the diagnosis of active tuberculosis, and further examination of sputum and imaging is required to clarify active tuberculosis.