Antifungal susceptibility testing of Candida spp. by relative growth measurement at single concentrations of antifungal agents.

The relative growth (percentage of growth relative to control growth) of 496 isolates representing six Candida species was assessed as a means of determining in vitro susceptibilities of the isolates in microdilution plate wells containing single concentrations of each of seven antifungal agents. The relative growth data were highly reproducible. With flucytosine and amorolfine they correlated well with MICs, but for an azole antifungal agent, terconazole, they did not correlate with MICs. Distributions of relative growth percentages for different Candida spp. showed significant differences in species susceptibility to individual agents. For example, C. albicans was less susceptible than the other species to amorolfine; C. parapsilosis isolates were particularly susceptible to terbinafine; and C. glabrata, C. guilliermondii, and C. krusei isolates were less susceptible than C. albicans to fluconazole and ketoconazole but equally susceptible as or more susceptible than C. albicans to itraconazole. Differential patterns of susceptibility to individual azole antifungal agents were noted for some individual strains as well as for Candida spp.