Caspofungin Etest Endpoint for Aspergillus Isolates Shows Poor Agreement with the Reference Minimum Effective Concentration

The Clinical and Laboratory Standards Institute (CLSI) M38-A2 reference broth microdilution (BMD) method for the antifungal susceptibility testing of filamentous fungi now includes guidelines for testing echinocandin activity using the minimum effective concentration (MEC) as the endpoint measurement. In this study, we compared the caspofungin Etest MIC on RPMI agar and Mueller-Hinton agar (supplemented with glucose and methylene blue [MGM]) to the BMD MEC for 345 clinical Aspergillus isolates, including A. flavus, A. fumigatus, A. nidulans, A. niger, and A. terreus. The essential agreement (±1 log2 dilution) of the Etest on MGM and RPMI agar with the reference BMD MEC was 18 and 26%, respectively. The geometric mean values for BMD MEC and MGM Etest were 0.137 and 0.024 μg/ml, respectively, and the geometric mean values for BMD and RPMI agar were 0.128 and 0.031 μg/ml, respectively. Comparatively, 91% of paired MGM and RPMI Etest results were within 2 log2 dilutions of each other and consistently produced clearly defined endpoints. In conclusion, the caspofungin Etest MIC, like the BMD MEC, is a reproducible endpoint but is markedly lower than the reference BMD. In anticipation of susceptibility breakpoint assignments, optimization studies will be required to improve the concordance of these two assays so that the potential for underreporting echinocandin resistance in Aspergillus is mitigated.