Skin testing in the diagnosis of tuberculosis

The tuberculin skin test is based on the expression of delayed hypersensitivity to tuberculoprotein following a mycobacterial infection. Unfortunately, the antigens used in such testing are crudely prepared and variable. Confounding the reliability of the test are the additional variables of the hosts' varying responses to the antigen and of the technical aspects of application and reading. These elements combine to make the tuberculin skin test a semiquantitative tool at best. In recent experience, approximately 20% of patients with active tuberculosis failed to manifest a positive skin test. Thus, caution should be exercised in using the skin test to rule out tuberculosis. When used to conduct epidemiological studies, group surveillance, or contact investigations, factors such as waning and small but significant reactions must be considered in interpreting the data.