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 caspofungin (MK-0991) of 726 isolates of Candida spp. 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. MICs were determined by Etest for all 726 isolates with RPMI agar containing 2% glucose (RPG) and were read after incubation for 48 h at 35 degrees C. The Candida isolates included Candida albicans (n = 486), Candida glabrata (n = 96), Candida tropicalis (n = 51), Candida parapsilosis (n = 47), Candida krusei (n = 11), Candida lusitaniae (n = 2), and Candida guilliermondii (n = 33). In addition, a subset of 314 isolates were also tested by Etest using Casitone agar (CAS) and antibiotic medium 3 agar (AM3). The Etest results obtained using RPG correlated well with reference MICs. Overall agreement was 94% with RPG, 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. When 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. In most instances (75%) where a discrepancy was observed between the Etest and the reference method, the Etest MIC was lower. The Etest method using RPG appears to be useful for determining caspofungin susceptibilities of Candida species.