Rapid flow-cytometric susceptibility testing of Candida species

Objectives: To develop a rapid flow-cytometric antifungal susceptibility test and to compare results with the standard methods. Methods: Reference and laboratory strains of Candida were tested for susceptibility to fluconazole and echinocandin by fluorescent flow cytometry using Acridine Orange as in dicator of viability. Flow cytometry results were compared with MICs as determined by macrodilution and/or Etest. Results: Seventy Candida strains were tested for susceptibility to fluconazole, and 74 strains for susceptibility to echinocandin. Minimal concentration of fluconazole causing 40% cell damage, as determined by flow cytometry, showed excellent association with MIC, as determined by other methods. The flow method, completed within 5 h, had excellent sensitivity and specificity to distinguish between sensitive, susceptible dose-dependent and resistant strains. The flow cytometry method for echinocandin was completed within 3 h, and minimal concentration causing 50% cell damage was associated with MIC as determined by macrodilution. Conclusions: Antifungal susceptibility testing by FACS is a reliable, rapid method for determining susceptibility of Candida to fluconazole and echinocandin. The method allows same-day results, assisting in the selection of appropriate antifungal therapy.