The test for tuberculosis, otherwise known as the tuberculin test or the PPD test, is done by injecting 5 tuberculin units directly under the skin of the forearm to produce a wheal or a bleb. Then, 48 to 72 hours after the injection, the skin should be examined for a raised, thickened local area, known as induration, and if there is any, measured in millimeters. The test is considered positive if any of the following criteria are met: if the induration in healthy individuals is equal to or greater than 15 millimeters or if there is blistering present; if the induration in individuals with kidney disease, diabetes, or one who has personal contact with someone who has active TB is at least 10 millimeters; if the induration in individuals who are immunocompromised is equal to or greater than 5 millimeters. For a more in-depth explanation of the interpretation of PPD test results, see here: http://www.medicinenet.com/tuberculosis_skin_test_ppd_skin_test/page2.htm#given
The tuberculin skin test signifies exposure to Mycobacterium tubercle bacilli. The skin on and around the injection will appear to be red, swollen induration greater than or equal to 15 mm is considered a positive skin test for tb.