Evaluation of p-nitro benzoic acid (pnb) inhibition test to differentiate Mycobacterium tuberculosis complex from non-tuberculous mycobacteria using microscopic observation of drug susceptibility (MODS) methodology

Abstract

Background: Microscopically observed drug susceptibility (MODS) assay relies on visualisation of characteristic serpentine cord for identification of Mycobacterium tuberculosis (MTB) complex. Since a few Non-tuberculous mycobacteria (NTM) also form cords or pseudocords, additional tests are required to obviate false positive result. Revised MODS platform by including p-nitro benzoic acid (PNB) in one of the wells is suggested to differentiate MTB from NTM.

Methods: 464 cultures positive by both MODS assay and reference solid LJ/liquid MGIT were included in the study. MODS protocol was modified by addition of a PNB well. Identification of MTB in reference culture methods was done by standard methods. Results obtained were compared with that of modified MODS assay.

Results: Overall concordance for species identification between MODS assay and reference solid LJ/liquid MGIT culture on the basis of visual detection of serpentine cords was 94.8% (95% Confidence interval- 92.3%-96.5%). When results of visual detection of serpentine cords were combined with PNB inhibition test, concordance was 100% (95% CI 98.9%-100%).

Conclusion: Based on results of our study, we recommend modification of MODS assay protocol by adding PNB to one well to differentiate MTB complex from NTM species.