Radioimmunoassay for detecting Mycobacterium tuberculosis antigen in cerebrospinal fluids of patients with tuberculous meningitis.

A biotin-avidin radioimmunoassay for detecting Mycobacterium tuberculosis antigen has been developed. The assay involves sandwiching mycobacterial antigens from sonicate preparations and cerebrospinal fluids between two antibodies produced in burros and rabbits. The reaction is amplified by using biotinylated antibody to rabbit IgG and 125I-labeled avidin. The assay has a sensitivity of 20 ng/ml and shows less than 5% cross-reactivity with six other mycobacteria. We studied patients with untreated tuberculous meningitis, patients with treated tuberculous meningitis, patients with nonbacterial meningitis, and patients with bacterial meningitis. Antigen was detectable in two of 56 control samples (40 ng/ml). Hence, samples with greater than or equal to 80 ng of antigen/ml were considered positive. In patients with untreated tuberculous meningitis, antigen levels ranged from 20 to 10,000 ng/ml, and 15 (79%) of 19 samples were positive. In the treated group, only two (10%) of 17 samples were positive. This test promises to be a rapid adjunct in the early diagnosis of tuberculous meningitis.