The purpose of the study was to evaluate the interlaboratory agreement of broth dilution susceptibility methods for five species of conidium-forming (size range, 2 to 7 m) filamentous fungi. The methods used included both macro-and microdilution methods that were adaptations of the proposed reference method of the National Committee for Clinical Laboratory Standards for yeasts (m27-P). The MICs of amphotericin B, fluconazole, itraconazole, miconazole, and ketoconazole were determined in six centers by both macro-and microdilution tests for 25 isolates of Aspergillus flavus, Aspergillus fumigatus, Pseudallescheria boydii, Rhizopus arrhizus, and Sporothrix schenckii. All isolates produced clearly detectable growth within 1 to 4 days at 35؇C in the RPMI 1640 medium. Colony counts of 0.4 ؋ 10 6 to 3.3 ؋ 10 6 CFU/ml (mean, 1.4 ؋ 10 6 CFU/ml) were demonstrated in 90% of the 148 inoculum preparations. Overall, good intralaboratory agreement was demonstrated with amphotericin B, fluconazole, and ketoconazole MICs (90 to 97%). The agreement was lower with itraconazole MICs (59 to 79% median). Interlaboratory reproducibility demonstrated similar results: 90 to 100% agreement with amphotericin B, fluconazole, miconazole, and ketoconazole MICs and 59 to 91% with itraconazole MICs. Among the species tested, the MICs for S. schenckii showed the highest variability. The results of the study imply that it may be possible to develop a reference method for antifungal susceptibility testing of filamentous fungi.Despite the lower volume of serious infections caused by filamentous fungi compared with that of serious infections caused by yeasts, the performance of antifungal susceptibility testing for these opportunistic pathogens is important in the clinical laboratory (8). Progress has been made in developing guidelines for the antifungal susceptibility testing of yeasts to deal with the standardization of different testing parameters such as inoculum preparation, medium composition and pH, length of incubation, and endpoint criteria (3,5,6,(9)(10)(11)(12). The standardization of these antifungal susceptibility testing steps has led to increased interlaboratory reproducibility which has opened the possibility of developing standards by adopting and tailoring these steps for antifungal susceptibility tests for the filamentous fungi. The first priority of the National Committee for Clinical Laboratory Standards (NCCLS) Subcommittee on Antifungal Susceptibility Tests was to develop a standard for the preparation of inoculum suspensions. Among four procedures evaluated, a spectrophotometric method was recommended as the procedure that gives the least variable results for the preparation of inoculum suspensions of yeast cells (11). This recommendation has been substantiated by ensuing collaborative studies of the subcommittee and other studies (2,3,5,6,12).The spectrophotometric method has been evaluated further in a single laboratory for the preparation of conidial suspensions of selected medically important filamentous fungi (1). In t...
