Antifungal Susceptibility Testing: A Primer for Clinicians

Abstract

Clinicians treating patients with fungal infections may turn to susceptibility testing to obtain information regarding the activity of different antifungals against a specific fungus that has been cultured. These results may then be used to make decisions regarding a patient's therapy. However, for many fungal species that are capable of causing invasive infections, clinical breakpoints have not been established. Thus, interpretations of susceptible or resistant cannot be provided by clinical laboratories, and this is especially true for many molds capable of causing severe mycoses. The purpose of this review is to provide an overview of susceptibility testing for clinicians, including the methods used to perform these assays, their limitations, how clinical breakpoints are established, and how the results may be put into context in the absence of interpretive criteria. Examples of when susceptibility testing is not warranted are also provided.

Keywords: antifungal; clinical breakpoint; susceptibility testing.

Figure 1.

Examples of various phenotypic susceptibility results for yeasts and molds. (A) shows the results of broth microdilution susceptibility testing per the Clinical and Laboratory Standards Institute methodology for fluconazole against Candida albicans, voriconazole against Aspergillus fumigatus, and amphotericin B against Purpureocillium lilacinum. (B) shows susceptibility results as measured by the YeastOne colorimetric assay against Candida species. (C) shows susceptibility results as measured by gradient diffusion for amphotericin B against A fumigatus, isavuconazole against Cryptococcus neoformans, and caspofungin against Candida glabrata. (D) shows the minimum effective concentration (MEC) results for micafungin against an Aspergilllus nidulans and an unidentified mold isolate. All testing was performed in the Fungus Testing Laboratory at the University of Texas Health Science Center at San Antonio. Red boxes in A, B, and D indicate minimum inhibitory concentration (MIC)/MEC values.