The TB skin test usually looks at the results within 48-72 hours. The TB skin test, also called the tuberculosis bacillus test and also known as the PPD test, involves injecting the TB virus into the skin and looking at the changes in the skin after 48-72 hours. If nodules appear, but they are small and less than 5 mm in diameter, the test is considered negative, or if no nodules appear and only redness is present, the test is also considered negative. If the nodules are larger than 5mm and smaller than 2cm in diameter, they are mostly considered positive. If the diameter is greater than 2 cm, it is usually a strong positive. A negative test indicates that the organism has not been infected with Mycobacterium tuberculosis, a weak positive test indicates that it has been infected with Mycobacterium tuberculosis, and a strong positive test indicates active tuberculosis.