Only identification of acid-fast bacilli of M tuberculosis in smear or culture in pleural effusion is completely diagnostic. However, a positive finding is rare, and the culture is time-consuming. Hence, tuberculous pleurisy is one of the major diagnostic problems in pulmonary disease.
Nassau et al first applied ELISA to the serodiag- nosis of tuberculosis. Since then, many reports have indicated that estimation of serum antibodies (especially IgG antibody) by ELISA has been helpful for diagnosis of tuberculosis. Daniel and Debanne recently summarized the state of the art and gave all- inclusive opinions regarding ELISA. In bronchial washing specimen with lung tuberculosis, or in cerebrospinal fluid with tuberculous meningitis, antibody estimation was also studied. Samuel et al studied antituberculous antibody and tuberculous antigen using radioimmunoassay technique and assessed them in ascitic and pleural fluid. There were significant statistical differences in both fuids between tuberculosis and malignancy. Banchuin et al attempted to evaluate anti-PPD IgG response in pleural fluid by ELISA, but they did not find any statistical difference between tuberculosis and malignancy. Thus, the assessment of ELISA in tuberculous pleurisy is controversial. But-we found significant differences in pleural effusions between tuberculosis and malignancy using anti-PPD IgG, IgA and IgM ELISA.
With the accuracy of anti-PPD antibody estimation for diagnosis of tuberculous pleural effusion, the sensitivity was low, but the specificity and the positive predictive value were high. The reason for the low sensitivity is unclear, but it might be due to the high tuberculous prevalence in Japan until the 1960s. In fact, IgG antibody reactive with the main PPD antigen band was positive in one malignant pleural effusion with our immunoblot analysis.
We selected PPD as the antigen to be used in ELISA. The PPD used in our study was derived from heat-killed cultures of M bovis. Though it is a relatively crude antigen preparation, PPD is less hazardous with regard to infection and readily available for use with ELISA. The PPD contains many heteromolecular components that form a complex antigenic structure. To purify strict species-specific antigens of M tuberculosis would be useful to distinguish tuberculosis from other diseases. Many investigators have described purified mycobacterial antigens. Daniel and Debanne reported that the sensitivity of ELISA was similar with all antigens used, but its specificity depended on the particular antigen employed. With highly purified antigens, ELISA would achieve high specificity.
Strockbine et al indicated that specific antigens seemed much more important than other ones in differential diagnosis with immunoblot analysis. To estimate the tuberculous-specific main antigen, we performed immunoblot of PPD and BCG whole cell fraction antigens to stain with IgG antibodies in pleural efiusions. The PAS stain of PPD antigen separated by SDS-PAGE demonstrated three heteromolecular sugar components. One of them corresponds with the main antigen, suggesting that the latter might be polysaccharide. Minden and Farr detected mucopolysaccharide antigens of M tuberculosus in immunized animal serum. Their finding in rabbit and monkey agrees with our observation, although the molecular weight of the antigen detected by antibody is different from that in human subjects. In cerebrospinal fluid, Chandramuki et al reported that M tuberculosis antigen recognized by IgG antibody was stained as a common broad band from 30 to 40 Kd region. This report supports our observation regarding the molecular weight.
The estimation of IgG antibody against PPD using ELISA and immunostain of PPD with pleural effusions had high specificity for diagnosis of tuberculous pleurisy. It is expected that strictly species-specific antigen of M tuberculosis will be readily available, so its application in ELISA and immunoblot analysis would assure high accuracy for diagnosis of tuberculosis.