The PPD test does not usually cause fever and is performed to detect previous or current tuberculosis infection. 48-72 hours of observation is required for the PPD test. Local redness of less than 5 mm is considered negative, while 5-10 mm is considered positive, and 10-20 mm is considered moderately positive. Greater than 20 mm or 2 cm, or the presence of skin rupture or blistering, also indicates a strong positive result and the possibility of a presenting tuberculosis infection. However, the PPD test is not the gold standard for confirming tuberculosis, and pathogenic testing is required to determine the occurrence of tuberculosis. If TB does occur, it is not the PPD test that causes the fever, but the disease itself that causes the fever.