Susceptibility of Candida species to fluconazole assayed by a disk diffusion method with automated reading versus a microdilution method

Abstract

In vitro susceptibilities of 290 isolates of Candida spp to fluconazole were evaluated by a new NCCLS M44-P agar disk diffusion method that was read and interpreted automatically by the BIOMIC image-analysis plate reader system. Disk test results were compared to results obtained by a modified NCCLS M27-A broth microdilution method using RPMI-1640 supplemented with 2% dextrose. Overall agreement between both methods was 90.0%. Category agreement between the broth and disk test results for susceptible, susceptible dose-dependent and resistant disk results were 99.6%, 19.05% and 52.17%, respectively. No very major discrepancies, 1.03% major discrepancies, and 8.97% minor discrepancies were observed between results of the two test methods. This analysis suggests that results from both methods correlate highly for Candida strains susceptible to fluconazole. The lower agreement between the two methods for resistant and susceptible dose-dependent isolates was due to strains near or on that breakpoint, different media (Mueller-Hinton vs RPMI), agar vs broth, 80% vs 50% endpoints, and trailing growth near the endpoints.