Antifungal susceptibility testing method for dermatophytes -determination of MIC and MFC values

In vitro antifungal susceptibility of pathogenic fungi is important information for physicians when selecting an appropriate antifungal drug, deciding the route of drug administration, surveying resistant strains, and so on. Although both the minimum inhibitory concentration (MIC) and the minimum fungicidal concentration (MFC) are well known endpoints of antifungal susceptibility, the MIC is by far the more highly referred in clinical laboratories. In fact, while methods for determining the MIC have been standardized in Japan and the West to ensure accuracy and reproducibility of the results, by contrast, scant attention has been paid to standardizing methods for determining the MFC. The same preference for MIC topical antifungal drugs for dermatophytes are concerned. In 1999, the Japanese Society of Medical Mycology published a new, standardized method of testing the MIC for dermatophytes and this has since been widely adopted. Nonetheless, the fact remains that the MFC is still determined using methods derived from antifungal pharmacology. Recently in Japan, however, the MFC of topical antifungal drugs has begun attracting more attention due to the development of new antifungal agents with fungicidal activity. These new developments call for improving our understanding of both the MIC and MFC as endpoints of antifungal susceptibility, and for standardizing methods for determining the MFC.The present paper has two objectives : first, to overview the MIC and MFC for topical drugs as endpoints of antifungal susceptibility; and second, to describe a novel test based on the standardized broth microdilution method combined with the trans-well system and neutral red, which we recently developed in our laboratory for directly measuring the MFC.