Immunodiagnosis of pulmonary tuberculosis by concomitant detection of antigen 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 Mycobacterium tuberculosis H37Ra and affinity purified polyclonal antibodies against Mtb EST were used to detect both antibodies and circulating antigen in the sera of patients and disease-free individuals. Indirect stick penicillinase ELISA system using Mtb EST-6 detected antigen-specific IgG antibody in 84% of sputum positive, 77% of sputum negative pulmonary tuberculosis patients and 7% of healthy and 11% of subjects with nontuberculosis diseases. Similarly, a sandwich penicillinase ELISA system using affinity purified anti Mtb EST antibodies detected circulating antigen in 83% and 61% of sputum positive and negative pulmonary tuberculosis subjects. In contrast only 24% of healthy and 18% of disease controls showed seropositivity. Antibody assay showed higher sensitivity and specificity (83% and 91% respectively) compared to antigen detection (sensitivity of 79% and specificity of 79%). However, by concomitant use of both assays it was possible to enhance the specificity of detection to 98%, though sensitivity was reduced marginally to 70%. The present study confirms the presence of both antigen and specific antibodies in the circulation during clinical disease and draws attention to the utility of Mtb EST-6 as a diagnostic marker of pulmonary tuberculosis.