The Clinical Laboratory Standards Institute (CLSI) approach to screening yeasts for antifungal resistance is time consuming and burdensome. There is a need for a more rapid and practical method. Previous studies using fluconazole in chromogenic agar have proven to be an effective method for identifying resistant yeasts. Objectives: Assess the utility of drug-impregnated solid medium to screen for voriconazole and caspofungin resistant yeasts and evaluate the utility of screening a variety of pathogenic yeasts for antifungal resistance by this method vs the CLSI reference standard. Methods: Yeasts were plated on solid agar media: CHROMagar Candida, RPMI, antibiotic medium 3, and Sabouraud's dextrose agar, each prepared according to manufacturer's directions with the addition of voriconazole at concentrations of 0, 1, and 4 µg/ml or caspofungin at concentrations of 0, 0.5, and 1 µg/ml. Two groups of 75 isolates were screened for voriconazole and caspofungin resistance following double-blind protocol using isolates that had been previously evaluated with the CLSI broth microdilution method. Results: Chromogenic agar exhibited abundant growth of susceptible yeasts on plates lacking drug, while only resistant yeasts grew to normal size on plates containing either drug. Compared to the CLSI standard, voriconazole resistance was correctly predicted for 6 of 12 (50%) isolates and susceptibility was correctly predicted for 63 of 63 (100%) isolates, whereas caspofingin resistance was correctly predicted for 17 of 18 (94%) isolates and susceptibility was correctly predicted for 56 of 57 (98%) isolates. However, comparisons of our screening method within one tube dilution of the CLSI broth microdilution method showed excellent agreement, correctly predicting 92% of voriconazole-resistant isolates and 100% of caspofungin-resistant isolates. Conclusion: Our solid agar drug resistance screening method appears to be a rapid and accurate method to identify antifungal-resistant yeasts. This work was supported by NIDCR, Grant #DE14318 for the CO STAR Program.

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