Tuberculin skin test
Prior tuberculosis infection can be detected by a positive Tuberculin skin test which is due to delayed hypersensitivity reaction. PPD is used as an antigen in the test. An immediate strength preparation of PPD, which 5 tuberculin units is used. The test is evaluated by measuring the diameter of induration on the skin test not the erythema. The diameter required to judge the test as positive varies depending on the status of the individual being tested. Induration of 15 mm or more is positive in a person exhibiting no known risk factors. Induration of 10 mm or more is positive in a person with high-risk factors, such as a homeless person, intravenous drug users, or a resident of nursing home. Induration of 5 mm or more is positive in a person who has deficient cell-mediated immunity (e.g., AIDS patients) or has been in close contact with a person with active tuberculosis by: Review of medical microbiology and immunology 15th edition by Warren levinson https://images.app.goo.gl/PHhAtA7zkVpx1hYw9