Evaluation of etest method for determining caspofungin (MK-0991) susceptibilities of 726 clinical isolates of Candida species

The performance of the Etest for testing the susceptibilities to caspofungi n (MK-0991) of 726 isolates of Candida spp. was assessed against the Nation al Committee for Clinical Laboratory Standards (NCCLS) microdilution broth method. The NCCLS method employed RPMI 1640 broth medium, and MICs were rea d after incubation for 48 h at 35 degreesC. MICs were determined by Etest f or all 726 isolates with RPMI agar containing 2% glucose (RPG) and were rea d after incubation for 48 h at 35 degreesC. The Candida isolates included C andida albicans (n = 486), Candida glabrata (n = 96), Candida tropicalis (n = 51), Candida parapsilosis (n = 47), Candida krusei (n = 11), Candida lus itaniae (n = 2), and Candida guilliermondii (n = 33). In addition, a subset of 314 isolates were also tested by Etest using Casitone agar (CAS) and an tibiotic medium 3 agar (AM3). The Etest results obtained using RPG correlat ed well with reference MICs. Overall agreement was 94% with R-PG, 82% with CAS, and 79% with AM3. When RPG was used, agreement ranged from 79% for C. parapsilosis to 100% for C. krusei, C. lusitaniae, and C. guilliermondii. W hen CAS was used, agreement ranged from 0% for C lusitaniae to 100% for C. glabrata. With AM3, agreement ranged from 0% for C. lusitaniae to 100% for C. guilliermondii. All three media supported growth of each of the Candida species. Etest results were easy to read, with sharp zones of inhibition. I n most instances (75%) where a discrepancy was observed between the Etest a nd the reference method, the Etest MIC was lower. The Etest method using RP G appears to be useful for determining caspofungin susceptibilities of Cand ida species.