Immunodiagnosis of tuberculous meningitis: rapid detection of mycobacterial antigens in cerebrospinal fluid by reverse passive hemagglutination assay and their characterization by Western blotting.

Tuberculous meningitis (TBM) is one of the commonest chronic infections of the central nervous system (CNS). Diagnosis of TBM has been a problem as it causes various clinical manifestations which can be confused with those of other chronic infections of the CNS such as neurocysticercosis (NCC), neurobrucellosis and cryptococcal meningitis, that are prevalent in many underdeveloped and developing countries. Differential diagnosis of TBM can be made by detecting circulating mycobacterial antigens in CSF by immunoassays. In this study, a reverse passive hemagglutination (RPHA) has been developed using rabbit antimycobacterial IgG for detection of circulating mycobacterial antigens in CSFs from chronic infections of the CNS in order to develop a rapid, simple, sensitive and cost-effective method. Circulating mycobacterial antigens were characterized by immunoblot assay. The sensitivity limit of RPHA was 400 ng ml(-1). RPHA was specific as antimycobacterial IgG did not show any reaction with porcine Cysticercus cellulosae which was used as a control antigen. RPHA could detect mycobacterial antigens in CSF at a sensitivity level of 94.11% with a specificity of 99.0%. Immunoblot analysis of RPHA positive CSFs revealed predominantly 30-32 kDa and 71 kDa antigens whilst 6, 86, 120, 96 and 110 kDa showed varied degree of reactivity. Antigens of masses 30-32 and 71 kDa were absent in culture filtrate of Mycobacterium tuberculosis H37Rv grown in Proskeur-Beck liquid medium. RPHA is a rapid, simple and sensitive immunological method with a long shelf life of 6-8 weeks if stabilized coated erythrocytes are stored at +4 degrees C. RPHA could be used as an additional immunodiagnostic tool in both differential diagnosis and prognosis of TBM. Immunoblot results indicate that 30-32 kDa and 71 kDa antigens are cell wall derived.