Evaluation of 80% inhibition standards for the determination of fluconazole minimum inhibitory concentrations in three laboratories

The purpose of this study was to evaluate 1:5 growth control dilutions (80% inhibition standards) to determine fluconazole minimum inhibitory concentration (MIC) end points in three laboratories. We tested 39 selected Candida species (in vitro susceptible: fluconazole MIC of ≤1 μg/ml, and resistant: fluconazole MIC of ≥8 μg/ml) and Cryptococcus neoformans isolates by broth macro- and microdilution procedures following the National Committee for Clinical Laboratory Standards proposed reference method for yeasts (M27-P). Macrodilution MIC₈₀% were the lowest drug concentrations with turbidity (growth) less than or equal to that of specific 1:5 dilutions of the growth control. Microdilution MICs-2 were the lowest drug concentrations in which there was prominent decrease of growth. A total of 1608 MICs were evaluated. C. krusei, C. parapsilosis, and C. tropicalis strains had reproducible fluconazole MICs by both tests (24 and 48 h). Fluconazole MIC₈₀% and MIC-2 end points were consistent at 24 h (with C. albicans) and more variable at 48 h. MICs of C. neoformans were more reproducible at 72 h than at 48 h by both tests. This study suggests that the determination of fluconazole MICs is dependent on the length of incubation and the yeast being tested, and that antifungal testing of the yeasts may be performed by either test.