Antifungal susceptibility testing: progress and future developments.

The establishment of a standardized broth reference method for antifungal susceptibility testing of yeasts has opened the door to a number of interesting and useful developments. The adaptation of the reference macrodilution method to a microdilution method has significantly increased the clinical utility of antifungal susceptibility testing, and both methods are now included in the NCCLS document M27-A. The publication of quality control limits for five antifungal agents, coupled with the establishment of interpretive MIC breakpoints for three agents, provides useful parameters to survey clinical isolates of Candida and other yeast species. Adaptations of the M27 microdilution method for testing molds has also proved feasible. These developments have made it possible for a number of recent studies designed to expand the capabilities of laboratories to perform antifungal susceptibility testing and to enhance our understanding of trends in antifungal susceptibility. The availability of reference methods also provides a useful touchstone for the development of commercial products that promise to be more user friendly and to further improve test standardization. Products in varying stages of development include two colorimetric microdilution methods (Sensititre and KPI) and the Etest stable gradient MIC method. Preliminary data indicate that these methods are viable alternatives to the reference method for testing of yeasts. Furthermore, Etest may also prove useful for testing molds. Future expectations for antifungal susceptibility testing includes improved ability to detect amphotericin B resistance, development of an NCCLS document for susceptibility testing of molds, and application of these methods for testing dermatophytes. Incorporation of antifungal susceptibility testing methods into the clinical trials of new antifungal agents will facilitate the establishment of clinical correlates and further enhance the clinical utility of antifungal susceptibility testing.