Methods for Antifungal Susceptibility Testing of the Cryptococcus neoformans/C. gattii Complex: Strengths and Limitations

Abstract

When method-dependent categorical endpoints are available, namely either BPs or ECVs, MICs could aid in selecting the best treatment agent(s). BPs can categorize an isolate as either susceptible or resistant while the ECVs/ECOFFs can distinguish the wild type (WT, no known resistance mechanisms) from the Non-WT (NWT, harboring resistant mechanisms). Our literature review focused on the Cryptococcus species complex (SC) and the available methods and categorization endpoints. We also covered the incidence of these infections as well as the numerous Cryptococcus neoformans SC and C. gattii SC genotypes. The most important agents to treat cryptococcal infections are fluconazole (widely used), amphotericin B, and flucytosine. We provide data from the collaborative study that defined CLSI fluconazole ECVs for the most common cryptococcal species or genotypes and modes. EUCAST ECVs/ECOFFs are not yet available for fluconazole. We have summarized the incidence of cryptococccal infections (2000–2015) where fluconazole MICs were obtained by reference and commercial antifungal susceptibility tests. This occurrence is documented all over the world and those fluconazole MICs are mostly categorized by available CLSI ECVs/BPs as “resistant” instead of non-susceptible strains, including those by the commercial methods. As expected, the agreement between the CLSI and commercial methods is variable because SYO and Etest data could yield low/variable agreement (<90%) versus the CLSI method. Therefore, since BPs/ECVs are species and method dependent, why not gather sufficient MICs by commercial methods and define the required ECVs for these species?