Comparative Evaluation of a New Fluorescent Carboxyfluorescein Diacetate-Modified Microdilution Method for Antifungal Susceptibility Testing of <i>Candida albicans</i> Isolates

Abstract

This report presents a fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method used for the susceptibility testing of Candida albicans to amphotericin B, fluconazole, ketoconazole, itraconazole, voriconazole, and flucytosine. Four different broth microdilution susceptibility testing methods were simultaneously evaluated at 24 and 48 h. The MICs determined using the CFDA-modified method (MIC(cfda)) were compared to those obtained by the standard broth microdilution method (MIC(visual)) and a procedure employing the indicator Alamar blue (MIC(alamar)). The reference MIC was determined visually as recommended by the NCCLS M27-A protocol, and then quantified spectrophotometrically following agitation (MIC(spec)). The CFDA-modified microdilution method was demonstrated to effectively determine the MICs for all the antifungal drugs tested at both 24 and 48 h. The results from both the MIC(spec) and MIC(cfda) methods yielded >80% agreement within +/-1 dilution and >90% agreement within +/-2 dilutions at 24 h in comparison to the reference MIC(visual) method, respectively. The trailing growth phenomenon that occurs with azole antifungal drugs and many strains of C. albicans did not inhibit the effectiveness of the MIC(spec) and MIC(cfda) methods. The MIC(spec) and MIC(cfda) methods shared 92.8% agreement within +/-1 dilution at 24 h and 87.6% agreement within +/-1 dilution at 48 h.