In Vitro Susceptibility Testing of Aspergillus spp.: Comparison of Etest and Reference Microdilution Methods for Determining Voriconazole and Itraconazole MICs

The performance of the Etest for voriconazole and for itraconazole susceptibility testing of 376 isolates of Aspergillus spp. was assessed in comparison with the National Committee for Clinical Laboratory Standards (NCCLS) proposed standard microdilution broth method. The NCCLS method employed RPMI 1640 broth medium, and MICs were read after incubation for 48 h at 35°C. Etest MICs were determined with RPMI agar containing 2% glucose and were read after incubation for 48 h at 35°C. The isolates included A. fumigatus , A. flavus , A. niger , A. terreus , A. versicolor , A. glaucus , A. nidulans , A. ustus , and A. sydowii . Overall agreement percentages between the Etest and microdilution MICs were 97.6% for voriconazole and 95.8% for itraconazole. Where a discrepancy was observed between Etest and the reference method, the Etest tended to give lower values with voriconazole and higher values with itraconazole. The Etest method using RPMI agar appears to be a useful method for determining the voriconazole and itraconazole susceptibilities of Aspergillus spp.