Susceptibility testing to second-line drugs and ethambutol by Genotype MTBDRsl and Bactec MGIT 960 comparing with agar proportion method

The incidence of multidrug-resistant tuberculosis (MDRTB) is increasing. Rapid detection of resistance to second-line drugs is essential for patient management and efficient control of tuberculosis. The aim of the present study was to assess the ability of the GenoType MTBDRsl DNA strip and the Bactec MGIT 960 assay to detect resistance to second-line drugs and ethambutol in multidrug-resistant clinical isolates using the agar proportion method as a reference technique. Twenty-six Mycobacterium tuberculosis complex isolates identified as multidrug-resistant on the basis of conventional drug susceptibility testing were retrieved from our laboratory archive (1992–2010) for evaluation. The susceptibility of these strains to second-line drugs and ethambutol was tested prospectively using MGIT 960 and GenoType MTBDRsl. The turnaround time for agar proportion, MGIT 960, and GenoType MTBDRsl were, respectively, 21 days, 8 days, and 8 h. Sensitivity values for MGIT 960 and GenoType MTBDRsl were, respectively, ethambutol (85.7, 28.6%), amikacin (50, 75%), and ofloxacin (50, 83.3%). Specificity values were, respectively, ethambutol (73.7, 89.5%), amikacin (72.7, 95.5%), and ofloxacin (100, 100%). Our data show that both methods have significant limitations and cannot replace conventional drug susceptibility testing. The results of resistance testing should be interpreted with caution and confirmed using the reference method.