Rapid Detection of Mycobacterium tuberculosis Resistance to Second-Line Drugs by Use of the Manual Mycobacterium Growth Indicator Tube System

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Abstract
The objective of this study was to evaluate the manual mycobacterium growth indicator tube (MGIT) system for the testing of Mycobacterium tuberculosis susceptibility to second-line drugs compared to the proportion method. One hundred eighty-eight M. tuberculosis isolates were tested for susceptibility to ofloxacin, kanamycin, ethionamide, and capreomycin by the manual MGIT, and results were compared to those obtained with the proportion method on 7H11 agar, considered a reference method. Results for ofloxacin and capreomycin were excellent, with 100% accuracy, and a result of 99.4% accuracy was achieved for kanamycin. For ethionamide, accuracy was lower, with a result of 86.7% compared to that of the proportion method. We proposed the following critical concentrations for the drugs: for ofloxacin, 2.0 μg/ml; for kanamycin, 2.5 μg/ml; for ethionamide, 5 μg/ml; and for capreomycin, 2.5 μg/ml. The time required to obtain results was an average of 8 days by the manual MGIT and 3 weeks by the reference method. Our results show that the manual MGIT is an accurate method for the rapid susceptibility testing of M. tuberculosis to second-line drugs. There is no need for a machine when using the manual MGIT, and results can be read with a simple UV lamp or with a semiquantitative reader, which considerably reduces the cost of the method.

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