Multicenter evaluation of a new disk agar diffusion method for susceptibility testing of filamentous fungi with voriconazole, posaconazole, itraconazole, amphotericin B, and caspofungin.

The purpose of this study was to correlate inhibition zone diameters, in millimeters (agar diffusion disk method), with the broth dilution MICs or minimum effective concentrations (MECs) (CLSI M38-A method) of five antifungal agents to identify optimal testing guidelines for disk mold testing. The following disk diffusion testing parameters were evaluated for 555 isolates of the molds Absidia corymbifera, Aspergillus sp. (five species), Alternaria sp., Bipolaris spicifera, Fusarium sp. (three species), Mucor sp. (two species), Paecilomyces lilacinus, Rhizopus sp. (two species), and Scedosporium sp. (two species): (i) two media (supplemented Mueller-Hinton agar [2% dextrose and 0.5 microg/ml methylene blue] and plain Mueller-Hinton [MH] agar), (ii) three incubation times (16 to 24, 48, and 72 h), and (iii) seven disks (amphotericin B and itraconazole 10-microg disks, voriconazole 1- and 10-microg disks, two sources of caspofungin 5-microg disks [BBL and Oxoid], and posaconazole 5-microg disks). MH agar supported better growth of all of the species tested (24 to 48 h). The reproducibility of zone diameters and their correlation with either MICs or MECs (caspofungin) were superior on MH agar (91 to 100% versus 82 to 100%; R, 0.71 to 0.93 versus 0.53 to 0.96 for four of the five agents). Based on these results, the optimal testing conditions for mold disk diffusion testing were (i) plain MH agar; (ii) incubation times of 16 to 24 h (zygomycetes), 24 h (Aspergillus fumigatus, A. flavus, and A. niger), and 48 h (other species); and (iii) the posaconazole 5-microg disk, voriconazole 1-microg disk, itraconazole 10-microg disk (for all except zygomycetes), BBL caspofungin 5-microg disk, and amphotericin B 10-microg (zygomycetes only).