Multicenter susceptibility testing of Candida clinical isolates to caspofungin by three different methods

Gobernado, M.; Canton, E.; Peman, J.; Baquero, F.; Sanchez Sousa, A.; Alvarez, M.; Rubio, C.; Gil, J.; Cisterna, R.; Martin Mazuelos, E.; Serrano, C.; Sanz Rodriguez, C.

Objective: To study the agreement between M27-A, M27-A+2% glucose (M27-A+g) and Sensititre Yeast One(R) (SYO) methods for yeast susceptibility testing to caspofungin. Methods: 25 blood culture Candida isolates (9 C. albicans, 5 C. parapsilosis, 3 C. krusei, 3 C. glabrata, 2 C. tropicalis, and 2 C. guilliermondii) were simultaneously tested in 5 centers using the same batches of reagents. ATCC 90028, ATCC 6258, and 6 genetically modified caspofungin-resistant Candida strains were used as controls. SYO plates had added caspofungin at concentrations ranging from 0.016 to 16 mg/L. Readings were performed at 24 and 48 h for SYO, 24 h for M27-A+g, and 48 h for M27-A. MIC was the concentration that inhibited 100% of control growth. Results: The overall interlaboratory agreements (+-2 log dilutions) were: M27-A vs SYO 24 h, 79.4% (range, 75-88.9); M27-A vs SYO 48 h, 87.1% (range, 75-97.2); and M27-A vs M27-A+g, 92.1% (range, 69.7-100). Intralaboratory agreements among methods were higher. Both M27-A+g and SYO methods yielded MIC values for the genetically modified caspofungin-resistant strains in agreement with previous reports (Douglas et al. 1997. AAC 41: 2471). Conclusions: The colorimetric method SYO is a fine, reproducible method to test susceptibility of Candida to caspofungin, showing optimal correlation with NCCLS M27-A when MIC readings were performed at 48 h. Both M27-A+g and SYO detected properly caspofungin-susceptible strains of Candida spp. Thus, SYO could be an alternative method in clinical laboratories for testing Candida susceptibility to caspofungin. Yet, its applicability to the detection of clinically relevant caspofungin-resistant strains remains to be determined.