The injection site for the tuberculin test should be intradermal on the flexor side of the forearm, where the tuberculin is injected into the skin, and the skin is observed for changes after 48-72 hours. The skin is negative if it appears red or if the nodules are very small, less than 5 mm in diameter. If nodules are present, they are positive if they are between 0.5 and 2 cm in diameter. A nodule is strongly positive if it is larger than 2 cm in diameter or if blisters are present. A strong positive test is helpful in the diagnosis of active tuberculosis. A positive test indicates that the tuberculosis bacillus has been infected and no vaccination is needed, while a negative test indicates that the tuberculosis bacillus has not been infected and BCG vaccination is needed for prevention.