Comparative evaluation of National Committee for Clinical Laboratory Standards broth macrodilution and agar dilution screening methods for testing fluconazole susceptibility of Cryptococcus neoformans

A simple screening method for fluconazole susceptibility of Cryptococcus neoformans using 2% dextrose Sabouraud dextrose agar (SabDex) with fluconazole was compared to the National Committee for Clinical Laboratory Standards (NCCLS) broth macrodilution method. By this method, fluconazole-susceptible C. neoformans isolates are significantly smaller on medium dth fluconazole than on fluconazole-free medium. Isolates with decreased susceptibility hale normal-size colonies on medium containing fuconazole. The 48-h NCCLS broth macrodilution MICs (NCCLS MICs) for isolates with normal-size colonies on 8- or 16-mu g/ml fluconazole plates were predicted to be greater than or equal to 8 or greater than or equal to 16-mu g/ml, respectively. On medium with 16 mu g of fluconazole per ml, all strains (84 of 84) for which the NCCLS MICs were <16 mu g/ml were correctly predicted, as were all isolates (7 of 7) for which the MICs were greater than or equal to 16 mu g/ml. Agar dilution appears to he an effective screening method for fluconazole resistance in C. neoformans.