The serodiagnosis of tuberculosis by enzyme-linked immunosorbent assay with tuberculin purified protein derivative

Tuberculin purified protein derivative (PPD) is a relatively crude antigen prepared from Mycobacterium tuberculosis and has species nonspecificity in immunological reaction. It is, however, more readily available than more highly purified materials. Therefore, the detection of IgG antibody to PPD was done by enzymed-linked immunosorbent assay (ELISA) and its diagnostic useful was evaluated in this study. The patients with active tuberculosis had significantly high titer of IgG antibody to PPD compared with healthy persons and the patients without tuberculosis (P less than 0.001). An upper limit of normal set (=cut-off titer) at 2 standard deviations above mean of logarithmic titers in 220 healthy adult subjects would result in positive test reactions on the sera from 78 of 100 patients with active tuberculosis. Although 8 of 39 with atypical mycobacteriosis would be positive, 6 of 7 were distinguished almost with tuberculosis by detecting antibodies to PPD from M. intracellulare and M. kansasii concurrently. The antibody titer increased after chemotherapy would be gradually reduced under the cut-off titer when culture of mycobacteria turned to negative and markers of inflammation became negative. In false-negative cases, 4 were patients with hypo-gammaglobulinemia, 6 were with fresh tuberculosis before chemotherapy, 2 were with negative CRP in all clinical course and 4 were with bacilli needed over 7 weeks culture. From these results, this assay is helpful in the diagnosis of tuberculosis and a useful marker for judgment of clinical improvement, although detection of antibody has its limitations.