Optimal susceptibility testing conditions for detection of azole resistance in Aspergillus spp.: NCCLS collaborative evaluation

The most important role of susceptibility testing is to identify potentiall y resistant isolates for the agent being evaluated, Standard testing guidel ines recently have been proposed for antifungal susceptibility testing of f ilamentous fungi (molds), This collaborative (eight centers) study evaluate d further newly proposed guidelines (NCCLS, proposed standard M38-P, 1998) and other testing conditions for antifungal susceptibility testing of Asper gillus spp. to itraconazole and three new triazoles, posaconazole CSCH56592 ), ravuconazole (BMS-207147), and voriconazole. MICs of itraconazole, posac onazole, ravuconazole, and voriconazole for 15 selected isolates of three s pecies of Aspergillus (A. fumigatus, A. flavus, and A. terreus) with well d ocumented in vitro, clinical, or animal data were determined in each center by using four medium formulations (standard RPMI-1640 [RPMI], RPMI with 2% dextrose, antibiotic medium 3 [M3], and M3 with 2% dextrose) and two crite ria of MIC determination (complete [MIC-0s] and prominent [MIC-2s] growth i nhibition) at 24, 48, and 72 h, The highest reproducibility (92 to 99%) was seen dth the standard RPM[I and M3 media. Moreover, the distinction betwee n itraconazole-resistant MICs of >8 mug/ml for clinically resistant strains ) and -susceptible (MICs of 0.03 to 1 mug/ml) isolates, as well as between a voriconazole-resistant laboratory mutant and other isolates (voriconazole MICs of 2 to >8 versus 0.12 to 2 mug/ml), was more consistently evident wi th the standard RPMI medium and when MIG-Os were determined at 48 h, These results provide further refinement of the testing guidelines for susceptibi lity testing of Aspergillus spp. and warrant consideration for inclusion in the future NCCLS document M38-A.