Summary
We examined the utility of agar dilution to screen yeasts for reduced susceptibility to several newer antifungal drugs including echinocandins and azoles. We compared agar dilution susceptibility screening with the CLSI method for Candida isolates. CHROMagar Candida medium was prepared with echinocandins and azoles added independently to agar prior to solidification. Assessment of resistance was based on growth characteristics, wherein decreased colony size in the presence of antifungal drugs was used as an indicator of resistance. Clinical Candida isolates of C. albicans, C. glabrata, C. parapsilosis C. tropicalis, C. guilliermondii, C. lusitaniae, C. rugosa and C. dubliniensis were screened for drug susceptibility. Overall, antifungal susceptibility against anidulafungin, caspofungin, micafungin, posaconazole and voriconazole, determined using CHROMagar agar dilution, were shown to be 96, 80, 94, 90 and 97% accurate, respectively, within one tube dilution of CLSI MICs for these clinical isolates. Categorical errors by percentage, had a broader range. Major errors with anidulafungin, caspofungin and micafungin were 3, 6 and 0%, respectively, while very major errors were 15, 55 and 38%. Major errors with posaconazole and voriconazole, respectively, were 12 and 0%, while very major errors were 0 and 22% respectively, compared to CLSI standards. Most of the assessment errors were with C. glabrata and C. parapsilosis. Agar dilution screening for drug susceptibility with the current panel of antifungal drugs is rapid, accurate and effective, however, determination of resistance or non-susceptibility in yeasts may be more problematic, and may be species dependent.