Tuberculin and BCG tests.

The tuberculin test is widely used for the diagnosis of tuberculosis in children as it is the only one that provides evidence of infection with M. tuberculosis. Of the tuberculins that are available, the most widely used are PPDS and PPDRT 23, in various strengths. A positive test indicates prior infection with the tubercle bacillus but not necessarily active disease. A positive test may also result from BCG vaccination though the response is usually less than 10 mm and tends to wane with time. In areas with a high prevalence of atypical mycobacteria in the environment, positive reactions may also be due to cross-reactivity. BCG has been recommended by some workers as a diagnostic test but suffers from the disadvantages of low specificity.