INTERLABORATORY EVALUATION OF ETEST METHOD FOR TESTING ANTIFUNGAL SUSCEPTIBILITIES OF PATHOGENIC YEASTS TO 5 ANTIFUNGAL AGENTS BY USING CASITONE AGAR AND SOLIDIFIED RPMI-1640 MEDIUM WITH 2-PERCENT GLUCOSE

An interlaboratory evaluation (two centers) of the Etest method was co nducted for testing the antifungal susceptibilities of yeasts. The MIC s of amphotericin B, fluconazole, flucytosine, itraconazole, and ketoc onazole were determined for 83 isolates of Candida spp., Cryptococcus neoformans, and Torulopsis glabrata. Two buffered (phosphate buffer) c ulture media were evaluated: solidified RPMI 1640 medium with 2% gluco se and Casitone agar, MIC endpoints were determined after both 24 and 48 h of incubation at 35 degrees C. Analysis of 3,420 MICs demonstrate d higher interlaboratory agreement (percentage of MIC pairs within a 2 -dilution range) with Casitone medium than with RPMI 1640 medium when testing amphotericin B (84 to 90% versus 1 to 4%), itraconazole (87% v ersus 63 to 74%), and ketoconazole (94 to 96% versus 88 to 90%). In co ntrast, better interlaboratory reproducibility was determined between fluconazole MIC pairs when RPMI 1640 medium rather than Casitone mediu m was used (96 to 98% versus 77 to 90%). Comparison of the flucytosine MICs obtained with RPMI 1640 medium revealed greater than 80% reprodu cibility. The study suggests the potential value of the Etest as a con venient alternative method for testing the susceptibilities of yeasts. It also indicates the need for further optimization of medium formula tions and MIC endpoint criteria to improve interlaboratory agreement.