Comparison of TaqMan^® Array Card and MYCOTB^TM with conventional phenotypic susceptibility testing in MDR-TB

BACKGROUND: Although phenotypic drug susceptibility testing (DST) is endorsed as the standard for second-line drug testing of Mycobacterium tuberculosis, it is slow and laborious.

METHODS: We evaluated the accuracy of two faster, easier methodologies that provide results for multiple drugs: a genotypic TaqMan^® Array Card (TAC) and the Sensititre^® MYCOTB^TM plate. Both methods were tested at three central laboratories in Bangladesh, Tanzania, and Thailand with 212 multidrug-resistant tuberculosis (MDR-TB) isolates and compared with the laboratories' phenotypic method in use.

RESULTS: The overall accuracy for ethambutol, streptomycin, amikacin, kanamycin, ofloxacin, and moxifloxacin vs. the phenotypic standard was 87% for TAC (range 70–99) and 88% for the MYCOTB plate (range 76–98). To adjudicate discordances, we re-defined the standard as the consensus of the three methods, against which the TAC and MYCOTB plate yielded 94–95% accuracy, while the phenotypic result yielded 93%. Some isolates with genotypic mutations and high minimum inhibitory concentration (MIC) were phenotypically susceptible, and some isolates without mutations and low MIC were phenotypically resistant, questioning the phenotypic standard.

CONCLUSIONS: In our view, the TAC, the MYCOTB plate, and the conventional phenotypic method have similar performance for second-line drugs; however, the former methods offer speed, throughput, and quantitative DST information.