Susceptibility Test Methods: Yeasts and Filamentous Fungi

Abstract

This chapter deals with systemic infections caused by Candida spp. other than C. albicans, Aspergillus spp., and other filamentous fungi (molds). The development of the Clinical and Laboratory Standards Institute (CLSI) reference method M27-A3 has improved the reproducibility of in vitro antifungal susceptibility data and facilitated the establishment of interpretive breakpoints for the triazoles fluconazole, itraconazole, and voriconazole and the echinocandins. Based on historical data and the pharmacokinetics of flucytosine, interpretive breakpoints for flucytosine and Candida spp. also have been established. Some correlation has been suggested between amphotericin B Minimal Inhibitory Concentration (MIC) results obtained by nonstandardized methods and clinical outcome. Unfortunately, most M27-A amphotericin B MICs for yeasts are within a very narrow range, precluding a clear discrimination between susceptible and potentially resistant isolates. The CLSI Subcommittee on Antifungal Susceptibility Testing has developed reference methods for broth macro- and microdilution susceptibility testing of yeasts and mold and more recently a disk diffusion method for yeasts and a proposed disk diffusion method for molds. The European Committee on Antifungal Susceptibility Testing (EUCAST) has developed a modified broth microdilution method for yeast and has developed breakpoints for itraconazole and fluconazole to be applied to this method. Some commercial methods have been approved for the antifungal susceptibility testing of Candida spp. and there is a move towards consensus in the standardized methodology employed in the United States and Europe. This should help to improve surveillance of resistance patterns worldwide and help in the development of universal clinically relevant breakpoints.