A TB skin test is done to see the results after 48-72 hours. The TB skin test, also known as the tuberculin test, or PPD test, involves injecting tuberculin toxin into the skin and observing the changes in the skin after 48-72 hours. If no nodules are present, or if the nodules are small and less than 0.5cm in diameter, the test is negative. If a nodule is present, it is positive if it is 0.5-2 cm in diameter, and strongly positive if the skin nodule is more than 2 cm in diameter. Strong positivity indicates the possibility of active tuberculosis. Positivity indicates infection with Mycobacterium tuberculosis, while negativity indicates no infection with Mycobacterium tuberculosis, and BCG vaccination is needed to prevent tuberculosis infection. If it is positive, no vaccine is needed. Strong positives sometimes have blisters in addition to nodules.