Diagnosis of active tuberculosis by immunological methods. 1. The effect of tuberculin reactivity and previous BCG vaccination on the antibody levels determined by ELISA.

Enzyme-linked immunosorbent assay (ELISA) was used to compare the levels of antibody to M. tuberculosis in the IgG, IgM and IgA classes in 107 patients with active pulmonary tuberculosis, 109 healthy tuberculin-positive and 34 healthy tuberculin-negative individuals and to assess the effect of previous BCG vaccination on these levels. Tuberculin-positive control subjects had higher levels of antibody than tuberculin-negative controls in the IgG class only. As the IgG class was the most discriminative from the diagnostic point of view, this phenomenon reduced the usefulness of the test. While 77% of patients had amounts of antibody above the level which included 97.5% of tuberculin-negative control subjects, only 62% of patients had such elevated levels when compared with the tuberculin-positive controls. The discriminatory power of the test was increased from 62 to 75% by assaying antibodies in both the IgG and IgA classes, but only two further patients were found to be positive when IgM antibody levels were also assayed. Previous BCG vaccination had no significant effect on the antibody levels.