Microscopic observation drug susceptibility assay in the diagnosis of multidrug-resistant tuberculosis

Early detection of multidrug-resistant Mycobacterium tuberculosis (MDR-TB) is of primary importance for both patient management and infection control. Optimal methods for identifying MDR-TB in a timely and affordable manner in resource-limited settings are not yet available.

To evaluate the performance of a low-technology but rapid drug susceptibility testing method, the microscopic observation drug susceptibility assay (MODS), in the concurrent detection of M. tuberculosis and its susceptibility to isoniazid (INH) and rifampin (RMP) directly from sputum specimens.

A total of 115 smear-positive TB patients admitted to Abbasia Chest Hospital, Cairo, Egypt, were simultaneously tested using MODS and the BACTEC MGIT 960 mycobacterial detection system for the detection of M. tuberculosis and the identification of MDR-TB samples.

MODS detected 112 (97.4%) samples and BACTEC MGIT detected 115 (100%). Of the 115 isolates tested for susceptibility to INH, RMP and MDR-TB, complete agreement between MODS and MGIT results was found among respectively 92.9%, 95.5% and 97.3% of samples. The sensitivity, specificity, and positive and negative predictive values of MODS in the detection of MDR-TB were respectively 95.3%, 98.6%, 97.6% and 97.1%. MODS results were obtained in a median of 8 days (range 5-21).

MODS is an optimal alternative method for timely and affordable identification of MDR-TB in resource-limited settings.