Proposed quality control ranges for 2-μg anidulafungin disk diffusion testing and ability of the method to detect strains with elevated echinocandin MIC values

Anidulafungin, a newer echinocandin with potent antifungal activity, was tested in an 8-laboratory M23-style quality control (QC) study to determine disk diffusion (DD) method ranges. Two lots of 2-μg disks (MAST and BD) and 3 lots of Mueller–Hinton agar (with 0.5 μg/mL methylene blue + 2% glucose) were used against 4 QC strains. Proposed ranges are Candida albicans ATCC 90028 at 24 to 39 mm (95.0% in range), Candida krusei ATCC 6258 at 20 to 35 mm (94.7%), Candida parapsilosis ATCC 22019 at 15 to 27 mm (95.5%), and Candida tropicalis ATCC 750 at 21 to 38 mm (94.6%); however, the ranges were very wide (13–18 mm), indicating between observer interpretive or reagent variation and compromised reproducibility. Challenge strains with elevated echinocandin MIC values (≥4 μg/mL, nonsusceptible) were tested (DD and MIC) and compared with isolates with reproducible anidulafungin MIC results at 2 μg/mL (susceptible). Discrimination between these nonsusceptible and susceptible (MIC, ≤2 μg/mL) yeast populations was very acceptable, for example, 97.6% categoric agreement by DD method.