In vitro susceptibility of Cryptococcus neoformans isolates to five antifungal drugs using a colorimetric system and the reference microbroth method

Minimum inhibitory concentrations (MICs) of amphotericin B, 5-flucytosine, fluconazole, itraconazole and ketoconazole were determined against 42 clinical isolates of Cryptococcus neoformans var. neoformans using the Alamar YeastOne colorimetric method and the NCCLS reference microdilution method. No strains with resistance to amphotericin B, itraconazole or ketoconazole were detected with either method. Using the reference method, the MICs of fluconazole were ≥64 mg/L, whereas using the colorimetric method all MICs were >16 mg/L. The MIC values of 5-flucytosine were also higher using the reference method (8-16 mg/L for 32% of isolates) compared with the colorimetric method. The percentage of agreement between the methods, using a difference of two dilutions, was 70.7% for itraconazole, 73.2% for amphotericin B, 80% for fluconazole, 88% for 5-flucytosine and 95% for ketoconazole. Overall, we conclude that for fluconazole and 5-flucytosine, in a low but not insignificant number of isolates, results with the two methods are discordant, some isolates being found sensitive with the colorimetric test, but resistant with the reference method.