Since numbers of drug-resistant Mycobacterium tuberculosis strains are on the rise, the simple classification into "susceptible" and "resistant" strains based on susceptibility testing at "critical concentrations" has to be reconsidered. While future studies have to address the correlation of phenotypic resistance levels and treatment outcomes, a prerequisite for corresponding investigations is the ability to exactly determine levels of quantitative drug resistance in clinical M. tuberculosis isolates. Here we have established the conditions for quantitative drug susceptibility testing for first- and second-line agents using MGIT 960 instrumentation and EpiCenter software equipped with the TB eXiST module. In-depth comparative analysis of a range of well-characterized susceptible and resistant clinical isolates has allowed us to propose conditions for testing and to develop criteria for interpretation.
