Application of a circulating antigen detection immunoassay for laboratory diagnosis of extra-pulmonary and pulmonary tuberculosis

Diagnosis of extra-pulmonary tuberculosis is often difficult to establish using standard methods. Recently, a 55-kDa mycobacterial antigen was identified in sera of individuals with pulmonary TB using a simple and rapid dot-ELISA based on monoclonal antibody (TB-55 mAb). Here, we have evaluated the application of the dot-ELISA for the detection of target antigen in sera of individuals with extra-pulmonary TB. The Western blot and indirect immunoperoxidase staining was used to identify the target TB antigen using the TB-55 mAb. The dot-ELISA was used to detect the target antigen in serum samples. The target antigen was identified at 55-kDa molecular weight in serum, ascitic fluid and CSF samples from individuals with extra-pulmonary TB. The purified antigen from these samples showed similar biochemical properties to the previously described antigen. The target antigen was localized in areas without caseous necrosis in lymph tissues. The dot-ELISA detected the target antigen in 90% sera of individuals with extra-pulmonary TB and in 87% sera of individuals with pulmonary TB with a specificity of 97% among control individuals. The detection of the 55-kDa antigen using dot-ELISA can be routinely employed to support clinical diagnosis of extra-pulmonary TB and pulmonary TB.