The purpose of antimicrobial susceptibility testing (AST) is to provide a confident prediction of the likely outcome of treatment. Methods calibrated to quantitative susceptibilities provide a means for diagnostic laboratories to deliver such predictions. In order to ensure comparable and reliable results, ISO 20776-1 advises that routine methods and diagnostic devices be evaluated against its reference broth microdilution method. While the three most commonly adopted disc diffusion methods in Australia differ marginally in selection of agar, disc potency and inoculum, CDS, CLSI and EUCAST can be related to each other because they are all calibrated against minimal inhibitory concentration (MIC).

The MICs of five international reference strains was determined by agar dilution on Sensitest and Mueller-Hinton agars using a Steer’s replicator and by the ISO broth microdilution method. Each technique of susceptibility testing has relative advantages and disadvantages and may be affected by reagent deterioration, minor technical deviations or subjectivity in end-point reading.

We compare the MIC results observed with three methods. Results were considered concordant if they fell within ± 1 doubling dilution of each other and within the published range for the organism.