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.

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.