IMMUNODIAGNOSIS OF PULMONARY TUBERCULOSIS BY CONCOMITANT DETECTION OFANTIGEN AND ANTIBODIES OF EXCRETORY-SECRETORY PROTEIN OF MYCOBACTERIUM-TUBERCULOSIS H37RA

With a view to diagnosing tuberculosis in populations in endemic areas , excretory-secretory antigen fraction (Mtb EST-6) of purified Mycobac terium tuberculosis H37Ra and affinity purified polyclonal antibodies against Mtb EST were used to detect both antibodies and circulating an tigen in the sera of patients and disease-free individuals. Indirect s tick penicillinase ELISA system using Mtb EST-6 detected antigen-speci fic IgG antibody in 84% of sputum positive, 77% of sputum negative pul monary tuberculosis patients and 7% of healthy and 11% of subjects wit h nontuberculosis diseases. Similarly, a sandwich penicillinase ELISA system using affinity purified anti Mtb EST antibodies detected circul ating antigen in 83% and 61% of sputum positive and negative pulmonary tuberculosis subjects. In contrast only 24% of healthy and 18% of dis ease controls showed seropositivity. Antibody assay showed higher sens itivity and specificity (83% and 91% respectively) compared to antigen detection (sensitivity of 79% and specificity of 79%). However, by co ncomitant use of both assays it was possible to enhance the specificit y of detection to 98%, though sensitivity was reduced marginally to 70 %. The present study confirms the presence of both antigen and specifi c antibodies in the circulation during clinical disease and draws atte ntion to the utility of Mtb EST-6 as a diagnostic marker of pulmonary tuberculosis.