A positive tuberculosis skin test indicates infection with Mycobacterium tuberculosis. The tuberculosis skin test is also called the tuberculin test, also known as the PPD test. The test is performed by injecting tuberculin toxin into the skin and observing changes in the skin 48-72 hours later. If no nodules appear in the skin, only redness, or even if nodules appear, but the diameter of the nodules is less than 5mm, the test is negative, which means that the person has not been infected with Mycobacterium tuberculosis and needs to be vaccinated against TB. If the diameter of the nodule is greater than 5 mm and less than 2 cm is positive, positive means that it has been infected with Mycobacterium tuberculosis and does not require vaccination. If the diameter is greater than 2 cm, or if there are blisters around the nodule, it is strongly positive, and strong positivity is the basis for the diagnosis of active tuberculosis.