Comparison of the Manual Mycobacteria Growth Indicator Tube and the Etest with the Method of Proportion for Susceptibility Testing of Mycobacterium tuberculosis

Background: Clinical microbiology laboratories should provide reliable results on susceptibility testing of Mycobacterium tuberculosis to different agents. Methods: The manual Mycobacteria Growth Indicator Tube (MGIT) and Etest were compared to the method of proportion (MOP) for susceptibility testing of 88 clinical isolates of M. tuberculosis against isoniazid (INH), rifampin (RIF), streptomycin (STR) and ethambutol (EMB). Isolates were recovered from different patients and were identified at species level by PCR and hybridization. Results: Resistance to INH was detected in 20.5, 29.5 and 12.5% of the isolates, followed by STR resistance (19.3, 26.1 and 1.1%), RIF (9.1, 4.5 and 5.7%) and EMB (2.3, 11.4 and 2.3%) by the MOP, MGIT and Etest, respectively. Sensitivity of the manual MGIT ranged from 37.5% for RIF resistance to 100% for EMB, while Etest sensitivity ranged from 5.9% for STR to 62.5% for RIF. Conclusions: MOP remains the method of choice, with the manual MGIT showing superior sensitivity at detecting resistance to INH, STR and EMB compared to the Etest.