Comparison of Etest and a tablet diffusion test with the NCCLS broth microdilution method for fluconazole and amphotericin B susceptibility testing of Candida isolates

Abstract

Three methods were compared for the susceptibility testing of yeast isolates to fluconazole and amphotericin B: two agar diffusion methods (Etest and a tablet diffusion test) and the National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method. Given as MIC50s (range), fluconazole endpoints were: for the 24 h broth microdilution test, 0.25 mg/L (0.06-32 mg/L); for the Etest, 0.38 mg/L (0.064-24 mg/L); and for the NCCLS broth microdilution test, 2 mg/L (0.06-≥ 64 mg/L). With breakpoints of <3 mg/L for susceptible and > 16 mg/L for resistant, the Etest and the 24 h microdilution test classified the isolates in agreement with the classification obtained by the NCCLS method. Results obtained by Etest were in closer NCCLS method than those obtained with the tablet test. Amphotericin B endpoints were lower for the 24 h microdilution and Etests than MICs obtained by the NCCLS broth microdilution method. Reproducibility was high for all tests; however, disadvantages of both diffusion tests were microcolonies in the inhibition zone and dependence on stringent standardization of inoculum.