Effectiveness of SD Bioline MPT 64 Test for Differentiating Mycobacterium tuberculosis Complex and Non-Tuberculous Mycobacteria from Liquid Culture Positive Isolates

Abstract

For proper management of TB, it is critical to distinguish Mycobacterium tuberculosis complex (MTBC) from other mycobacteria. The traditional biochemical methods are slow and error prone, require setting up of multiple tests. To differentiate MTBC from Non Tuberculous Mycobacteria (NTM) a new immunochromatographic test kit that is rapid “SD Bioline TB Ag MPT64”has been evolved by Standard Diagnostics, South Korea. It detects MPT64 antigen specific for MTBC.120 liquid culture positive isolates of mycobacteria were enrolled for the study and tested by SD Bioline TB Ag MPT64 test kits. For positive control H37RV standard strain was used. These were further processed for GenoType^. Mycobacterium CM/AS assay for further discrimination to the species level. Among the 120 isolates initially negative with MPT64 and classified as NTM on further testing by CM/AS, 13(10.83%)  were identified as MTBC. The liquid culture tube growth of these 13 isolates after one month was repeated for MPT64, 8/13 (61.53%) gave positive results and 5/13 (38.46%) remained negative. MPT64 test may be negative in 10.83% cases even when the isolate is MTBC on initial testing of Acid fast Bacilli (AFB) smear positive liquid culture.  Moreover, 8(6.66%) isolates reported to be positive on prolonged incubation (28 days) indicating that the amount of MPT64 antigen secreted may be low in some isolates. Hence care should be taken to identify MPT64 negative isolates as some MTB may be missed otherwise.