Susceptibility Test Methods: Yeasts and Filamentous Fungi

While antifungal drug resistance has not proved to be as problematic as that experienced with antibacterial agents, both intrinsic and emergent resistance are encountered and antifungal susceptibility testing can help in the guidance of prescribing practices. Moreover, susceptibility testing is of great importance in detecting intrinsic resistance in a species and therefore establishing the spectrum of activity of new and developmental agents. A great deal of progress has been achieved in the field of antifungal susceptibility testing with both yeasts and filamentous fungi since testing began in earnest in the early 1980s. Standardized broth macrodilution and microdilution methods are available for testing moulds and yeasts, as are standardized disk diffusion methods for systemically active antifungal drugs. Progress is also being made in establishing the relationship between test results and patient responses to therapy in varied clinical settings and with many of the currently available antifungal agents. There has been a strong move towards consensus in the standardization of the methodology employed and the principles by which breakpoints are selected in the USA and Europe, which has meant that internationally agreed epidemiological cutoff value breakpoints can be applied for some drug–fungus combinations. This chapter addresses the methodological issues surrounding antifungal susceptibility testing and breakpoint setting.