Evaluation of enzyme immunoassays using purified protein derivative (PPD) and its pooled fractions 3 and 4 for diagnosis of pulmonary tuberculosis

Abstract

A critical evaluation of two enzyme immunoassays (EIAs) for diagnosis of pulmonary tuberculosis is reported. Purified protein derivative (PPD) or its pooled fractions 3 and 4 were used as antigens for detection of antibodies in sera from 53 patients with active pulmonary tuberculosis and 10 normal healthy individuals. The cut-off point for each EIA was based on the absorbance (mean + 3 SD) of normal sera with the respective antigens. All the normal sera were negative in both the assays. The positivity of tuberculosis patients in either assay was 86.8 per cent. Thus, for serodiagnosis of tuberculosis fractions 3 and 4 of PPD could serve as a good substitute for whole PPD. Sera from 45 leprosy patients were also analysed to assess the specificity of the EIAs. The mean reactivity of tuberculoid leprosy sera was comparable to that of normal sera. The ratio of the mean absorbance of lepromatous leprosy (LL) sera and normal sera was 16.73 with PPD, in comparison to 21.95 for pooled fractions 3 and 4. Out of 10 LL patients 9 (90%) were positive with fractions 3 and 4, in comparison to 10 (100%) with PPD. 71.1 per cent of leprosy patients belonging to different categories were positive in assay based on PPD in comparison to 64.4% in EIA using fractions 3 and 4. The high false positivity of leprosy sera in an assay designed for detection of pulmonary tuberculosis has immense implications in interpretation of results of the assay for diagnostic and epidemiological purposes.