Improved diagnosis of pulmonary tuberculosis by detection of free and immune complex-bound anti-30 kDa antibodies.

The 30 kDa secreted antigen of Mycobacterium tuberculosis was purified to homogeneity by serial chromatography, and enzyme linked immunosorbent assay (ELISA) was used to evaluate its diagnostic value in patients with pulmonary tuberculosis. The immunoglobulin (Ig) antibodies G, A, and M were estimated in the two groups: patients who were smear- and culture-positive (S+C+) for pulmonary tuberculosis and normal healthy subjects (NHS). Sensitivity of 67.4%, 14.8%, and 14.3%, with the specificity of 99%, 96.7%, and 92% were obtained for the 3 isotypes respectively. Combination of the results of IgG and IgA increased the sensitivity to 71%, with 97% specificity. Polyethylene glycol precipitation of the circulating immune complexes (CIC) in sera was carried out. The CIC bound antibodies offered a sensitivity of 92.5%, 85.4%, and 68.7%, respectively for the S+C+, S-C+, and S-C- patients, while the specificity was 96.6%. Thus CIC-bound antibodies promise to be a better diagnostic tool in the detection of tuberculosis.