In vitro susceptibility testing of filamentous fungi: Comparison of Etest and reference microdilution methods for determining itraconazole MICs

The performance of the Etest for itraconazole susceptibility testing of 50 isolates of filamentous fungi was assessed in comparison with the National Committee for Clinical Laboratory Standards (NCCLS) proposed standard micro dilution broth method. The NCCLS method employed RPMI 1640 broth medium, an d MICs were read after incubation for 48 h at 35 degrees C, Etest MICs were determined with RPMI agar containing 2% glucose and with Casitone agar and were read after incubation for 24 h (Aspergillus spp, and Rhizopus spp.) a nd 48 h (all species except Rhizopus spp,) at 35 degrees C, The isolates in cluded Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, Asperg illus terreus, Fusarium spp,, Pseudallescheria boydii, Rhizopus spp,, Paeci lomyces variotii, and an Acremonium sp. Overall agreement between Etest and microdilution MICs was 96% with RPMI agar and 80% with Casitone agar, The agreement was 100% for all species except Rhizopus spp, (83%) and Paecilomy ces varioti (0%) with RPMI agar, When Casitone agar was used, the agreement ranged from 50% with Rhizopus spp, to 100% with Fusarium spp,, P, boydii, P. varioti, and an Acremonium sp. Notably, for Aspergillus spp., the agreem ent between itraconazole Etest MICs read at 24 h and reference microdilutio n MICs read at 48 h was 100% with both RPMI and Casitone agar, Both media s upported the growth of all filamentous fungi tested. Where a discrepancy wa s observed between Etest and the reference method, the Etest MIC was genera lly higher. The Etest method using RPMI agar appears to be a useful method for determining itraconazole susceptibilities of Aspergillus spp, and other filamentous fungi.