Evaluation of the Etest method for determining fluconazole susceptibilities of 402 clinical yeast isolates by using three different agar media

Abstract

The performance of the Etest for fluconazole susceptibility testing of 402 yeast isolates was assessed against the National Committee for Clinical Laboratory Standards (NCCLS) microdilution broth method. The NCCLS method employed RPMI 1640 broth medium, and MICs were read after incubation for 48 h at 35 degrees C. Etest MICs were determined with RPMI agar containing 2% glucose (RPG), Casitone agar (CAS), and Mueller-Hinton agar (MHA) and were read after incubation for 48 h at 35 degrees C. The yeast isolates included Candida albicans (n = 161), Candida glabrata (n = 41), Candida tropicalis (n = 35), Candida parapsilosis (n = 29), Candida krusei (n = 32), Candida lusitaniae (n = 31), Candida species (n = 19), Cryptococcus neoformans (n = 40), and miscellaneous yeast species (n = 14). The Etest results correlated well with reference MICs. Overall agreement was 94% with RPG, 97% with CAS, and 53% with MHA. When RPG was used, agreement ranged from 89% for Candida spp. to 100% for C. krusei. When CAS was utilized, agreement ranged from 93% for Cryptococcus neoformans to 100% for C. tropicalis, C. parapsilosis, C. lusitaniae, Candida spp., and miscellaneous yeast species. With MHA, agreement ranged from 17% for C. parapsilosis to 90% for C. krusei. Both RPG and CAS supported growth of all yeast species, whereas growth on MHA was comparatively weaker. Etest results were somewhat easier to read on CAS. The Etest method using either RPG or CAS, but not MHA, appears to be a viable alternative to the NCCLS reference method for determining fluconazole susceptibilities of yeasts.

FIG. 1

Fluconazole (FL) Etest reading patterns for C. albicans. (A) Growth of microcolonies inside the entire inhibition zone (ellipse); MIC, 0.38 μg/ml. (B) Clear ellipse on Casitone agar; MIC, 0.5 μg/ml. The numbers on the scale correspond to the fluconazole concentrations on the strip (in micrograms per milliliter).

FIG. 2

Fluconazole (FL) Etest reading patterns for C. glabrata. A resistant subpopulation appears as macrocolonies within the ellipse on Casitone agar. MIC, >256 μg/ml. The numbers on the scale correspond to fluconazole concentrations on the strip (in micrograms per milliliter).