Fluconazole and voriconazole multidisk testing of Candida species for disktest calibration and MIC estimation

Fluconazole and voriconazole MICs were determined for 114 clinical Candida isolates, including isolates of Candida albicans, Candida glabrata, Candida krusei, Candida lusitaniae, Candida parapsilosis, and Candida tropicalis, All strains were susceptible to voriconazole, and most strains were also su sceptible to fluconazole, with the exception of C, glabrata and C, krusei, the latter being fully fluconazole resistant. Single-strain regression anal ysis (SRA) was applied to 54 strains, including American Type Culture Colle ction reference strains. The regression lines obtained were markedly differ ent for the different Candida species. Using an MIC limit of susceptibility to fluconazole of less than or equal to8 mug/ml, according to NCCLS standa rds, the zone breakpoint for susceptibility for the 25-mug fluconazole disk was calculated to be greater than or equal to 18 mm for C, albicans and gr eater than or equal to 22 mm for C, glabrata and C, krusei. SRA results for voriconazole were used to estimate an optimal disk content according to ra tional criteria. A 5-mug disk content of voriconazole gave measurable zones for a tentative resistance limit of 4 mug/ml, whereas a 2.5-mug disk gave zones at the same MIC level for only three of the species, A novel SRA modi fication, multidisk testing, was also applied to the two major species, C, albicans and C. glabrata, and the MIC estimates were compared with the true MICs for the isolates, There was a significant correlation between the two measurements, Our results show that disk diffusion methods might be useful for azole testing of Candida isolates. The method can be calibrated using SRA. Multidisk testing gives direct estimations of the MICs for the isolate s.