Amphotericin B and Fluconazole Susceptibility Test of Candida Species: Comparison of Broth Microdilution Method and Agar Dilution Method

Chae, Jeong Don; Kang, Jung Oak; Eom, Jeong In; Park, Ile Kyu; Choi, Tae Yeal

Abstract

BACKGROUND: The rising incidence of fungal infections and the increasingly frequent use of antifungal agents have intensified the need for practical and reliable antifungal susceptibility test methods. In the present study, the minimal inhibitory concentration(MIC) distribution of Candida species was investigated and the agar dilution method was compared with the National Committee for Clinical Laboratory Standards(NCCLS) "reference" broth microdilution method for antifungal susceptibility testing.
METHODS
A total of 116 clinical isolates of Candida species from patients at Hanyang University Kuri Hospital were studied from October 1997 to July 1999, and the MICs of Candida species were evaluated against amphotericin B and fluconazole by the NCCLS method and the agar dilution method.
RESULTS
There were no differences in the MIC50 and MIC90 of Candida albicans and Candida tropicalis against amphotericin B between the two methods, but the MIC50 of C. albicans against fluconazole was higher in the agar dilution method than in the broth microdilution method. The resistant rate of C. albicans against fluconazole was 20.8% in the broth microdilution method and 33.8% in the agar dilution method. For C. tropicalis, 31.3% were resistant to fluconazole in the broth microdilution method and 25.0% in the agar dilution method. The agreement of C. albicans and C. tropicalis between the agar dulution method and the broth microdilution method within one doubling dilution of the microdilution reference were 88.8% for amphotericin B and 34.4% for fluconazole.
CONCLUSIONS
The MICs of amphotericin B showed good agreement between the agar dilution and the broth microdilution method. Therefore, resistant strains could easily be detected by screening with agar of 2 g/mL concentrations. However, in the case of fluconazole, the agreement between the two methods was low and the trailing effect could not be ruled out in agar dilution method with some strains, indicating the need for further studies in this area.