The TB skin test is also called the tuberculosis bacillus test, also known as the PPD test. It is not normal for the skin to turn purple during the TB skin test. Normally, erythema or nodules or blisters may appear, but not purple. The purple color may be due to bruising after a small amount of local subcutaneous bleeding during the skin test, and the bruising will gradually absorb and dissipate after a few days. After the TB skin test, it is important to keep the needle site dry and clean. If an erythema or nodule appears, it is considered negative if the nodule is relatively small, less than 5 mm in diameter, indicating that the person has not been infected with Mycobacterium tuberculosis. If a nodule appears at the site of the needle, a nodule with a diameter greater than 5 mm and less than 2 cm is considered positive, indicating that there has been an infection with Mycobacterium tuberculosis. If the nodule is larger than 2 cm or blisters are present at the site of the needle prick, it is a strong positive, and a strong positive is helpful in confirming the diagnosis of active tuberculosis.