Multicentre evaluation of an automated BACTEC 960 system for susceptibility testing of Mycobacterium tuberculosis.

SETTING: Three mycobacteria reference laboratories in the south-eastern part of Brazil. OBJECTIVE: To evaluate the automated Mycobacteria Growth Indicator Tube (MGIT) for drug susceptibility testing of Mycobacterium tuberculosis. DESIGN: Performance of the automated BACTEC MGIT 960 (M960) system for testing M. tuberculosis susceptibility to streptomycin (SM), isoniazid (INH), rifampicin (RMP) and ethambutol (EMB) was evaluated with 95 clinical isolates and compared to the results of the radiometric BACTEC 460TB (B460) system, the proportion method (PM), and the resistance ratio method (RRM). Judicial susceptibility profiles of 88 isolates were defined based on two or more concordant results among B460, PM and RRM, and used as a reference for comparison with M960 results. RESULTS: Agreement rates between M960 and conventional methods were 95.2% with B460, 96.6% with the PM and 93.4% with the RRM. The lowest agreement rates were obtained for SM with the RRM and for EMB with B460. When comparing M960 with judicial susceptibility profiles, the agreement rate was 97.9%. The agreement rates obtained for INH and RMP were 99.2% and for SM and EMB they were 96.2% and 96.9%, respectively. The mean time to reporting the M960 results was 6.9 days. CONCLUSION: M960 offers great improvements when compared to the proportion and resistance ratio methods and would benefit patient treatment.

Multicenter study of MTT and resazurin assays for testing susceptibility to first-line anti-tuberculosis drugs.

OBJECTIVE: A multicentre evaluation was performed to assess two rapid low-cost methods, MTT (3-[4.5-dimethylthiazol-2-yl]-2.5-diphenyltetrazolium bromide) and resazurin assays, for testing the susceptibility of Mycobacterium tuberculosis to the first-line anti-tuberculosis drugs rifampicin (RMP), isoniazid (INH), ethambutol (EMB) and streptomycin (SM). METHODS: Thirty coded M. tuberculosis strains were sent to seven laboratories located in Latin America, representing six countries. Each site performed the colorimetric assays, MTT and resazurin, blind for the first-line drugs RMP, INH, EMB and SM. The minimum inhibitory concentration results obtained were compared to the conventional proportion method on Lowenstein-Jensen medium. RESULTS: After establishing the breakpoint concentrations, excellent results were obtained for RMP, INH and EMB, with levels of specificity and sensitivity of between 96% and 99%. CONCLUSION: MTT and resazurin assays are promising, accessible new alternative methods for middle- and low-resource countries that need low-cost methods to perform rapid susceptibility testing of M. tuberculosis to key anti-tuberculosis drugs.