Comparison of three methods for in vitro susceptibility testing of Candida species with flucytosine

Abstract

Optimal methods for susceptibility testing of Candida spp. with flucytosine have not been determined. Breakpoints were recommended in 1984, but never validated. In this study, we compared the 1984 recommended macrodilution broth method (using an 80% endpoint) with a modification of the more recent NCCLS-recommended microdilution broth method with three endpoints-spectrophotometric 50% and 80% and a no growth endpoint determined by eye. NCCLS and British Society for Medical Mycology (BSMM) breakpoints were also compared. One hundred and fifty isolates comprised of Candida albicans, Candida tropicalis, Candida krusei, Candida glabrata, Candida parapsilosis and Candida lusitaniae were tested. Reproducibility was excellent. For C. albicans (n = 65), the correlation between tests was excellent (>75%), with few major discrepancies (<5%). For C. tropicalis (n = 27), correlation was good (59%), but there were a small number of major discrepancies (up to 11%, depending on breakpoint used). Results by the broth macrodilution method were generally higher than both microdilution methods for C. glabrata (n = 16; correlation of 18.8%), but only one major discrepancy was seen. Ten of the 11 C. parapsilosis isolates tested were susceptible by all methods, regardless of breakpoint chosen, with a correlation of 18.2%, but no major discrepancies were seen. A correlation between all methods (50%) was seen with C. lusitaniae (n = 10), with many isolates resistant or intermediate. In contrast, correlation between methods for C. krusei was poor (<5%); NCCLS microtitre modification produced results that were classified as intermediate or resistant, regardless of the breakpoint used. The methodology for susceptibility testing C. albicans is robust. Additional work to optimize susceptibility testing with flucytosine is necessary for non-albicans Candida species, especially C. krusei.