Multicenter laboratory validation of susceptibility testing of Mycobacterium tuberculosis against classical second-line and newer antimicrobial drugsby using the radiometric BACTEC 460 technique and the proportion method with solid media

In a large multicenter study involving six major study sites in the United States, Canada, and Europe, the susceptibilities of 272 Mycobacterium tuber culosis strains to classical second-line antituberculosis (anti-TB) drugs ( capreomycin, cycloserine, ethionamide, and kanamycin) and newer compounds ( amikacin, clofazimine, ofloxacin, and rifabutin) were determined by the rad iometric BACTEC 460 procedure and the conventional proportion method on Mid dle brook 7H10 agar, Previously established critical concentrations for cla ssical second-line anti-TB drugs were compared with several concentrations in liquid medium to establish equivalence. MICs of newer compounds determin ed in liquid medium were either the same or up to four times lower than tho se determined in agar medium. After establishing critical concentrations (b reakpoints) in the extended testing of clinical isolates, we obtained an ex cellent overall correlation between the two systems, with no errors with am ikacin, kanamycin, and ofloxacin and very few major or very major errors wi th the other drugs; however, for cycloserine, no breakpoint concentration c ould be recommended due to repeatedly inconsistent results by both methods. Based on these data we conclude that: the BACTEC 460 procedure is a simple and rapid method requiring 4 to 8 days on average to generate accurate ant imicrobial susceptibility testing (AST) results for eight anti-TB drugs oth er than those considered primary ones. These data not only fill a major gap of knowledge regarding the critical test concentrations of secondary anti- TB drugs but also provide a baseline for future evaluations of M. tuberculo sis AST with the more recently developed, nonradiometric broth-based cultur e systems.