Antimycobacterial antibody levels in pleural fluid as reflection of passive diffusion from serum

The objective of this study was the prospective evaluation of the relationship between serum and pleural fluid antibody levels to mycobacterial antigens and their role in the diagnosis of tuberculous pleuritis. The setting was a tertiary care medical center. Thirteen patients with tuberculous pleuritis and 53 control subjects with pleural effusion (22 with carcinoma, 17 with cardiac failure, and 14 with empyema or parapneumonic effusion) were studied. The level of IgG was measured by ELISA. The median titers of antibody to both Mycobacterium tuberculosis and M avium were significantly higher in the serum and pleural fluid of the patients with tuberculosis than in the control patients. There was a very close relationship between the levels of M tuberculosis (r = 0.95) and M avium (r = 0.94) antibodies in the serum and pleural fluid. We concluded that the levels of antimycobacterial IgG in pleural fluid, adjusted to constant protein concentration, are very closely related to the serum levels. Therefore, these antibodies in the pleural fluid probably result from passive diffusion from serum and not local production. Measurement of pleural fluid antibody levels will not add diagnostic sensitivity or specificity to that achieved with serodiagnosis.