Utility of Microscopic Observation of Drug Susceptibility (MODS) assay for Mycobacterium tuberculosis in resource constrained settings.

Introduction: Multidrug Resistant Tuberculosis (MDR TB) is a global health problem. Conventional techniques or automated systems for diagnosis and drug susceptibility testing are either comparatively slow or costly. Microscopic Observation Drug Susceptibility [MODS] assay is a simultaneous detection and direct drug susceptibility test [DST] method which relies on the characteristic growth of Mycobacterium tuberculosis (MTB) in a liquid medium.

Aim: Comparison between MODS assay and culture on Lowenstein Jensen (LJ) medium with respect to; (i) detection of mycobacterial growth and time taken for culture positivity (ii) to compare concordance of susceptibility results of MTB isolate by MODS with proportion method using LJ medium.

Method: A prospective study was carried out on 171 acid fast smear positive sputum specimens. The decontaminated sediment was used for culture and DST using LJ medium (proportion method) and MODS assay plates containing supplemented Middlebrook 7H9 broth with and without critical concentrations of isoniazid and rifampicin.

Results: Median time to growth and DST using MODS assay was 10 days and that with LJ medium was 24 and 66 days respectively. The sensitivity and specificity of MODS assay was 100%. All the isolates were characterized as MTB. MODS demonstrated 98.8% and 99.4% concordance for isoniazid and rifampicin respectively and 100% for MDRTB. Positive and negative predictive value for MDRTB was 100%.

Conclusion: MODS assay offers a rapid, simple, economical and feasible method for simultaneous culture and DST of MTB. Utility of MODS needs to be ascertained in extrapulmonary TB cases.