Novel Fluorescent Broth Microdilution Method for Fluconazole Susceptibility Testing of Candida albicans

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Abstract
A comparative evaluation of the reference National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method with a novel fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method for the susceptibility testing of fluconazole was conducted with 68 Candida strains, including 53 Candida albicans , 5 Candida tropicalis , 5 Candida glabrata , and 5 Candida parapsilosis strains. We found trailing endpoints and discordant fluconazole MICs of C. albicans strains. These strains satisfy the definition of the low-high MIC phenotype. All 12 low-high phenotype strains were correctly shown to be susceptible at 48 h with the CFDA-modified microdilution method. For the 41 non-low-high phenotype C. albicans strains, the CFDA-modified microdilution method yielded 97.6% (40 of 41 strains) agreement within ±1 dilution at 24 h compared with the reference method and 92.7% (38 of 41 strains) agreement within ±1 dilution at 48 h compared with the reference method. The five strains each from C. tropicalis , C. glabrata, and C. parapsilosis that were tested showed 100% agreement within ±2 dilutions for the two methods being evaluated.

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