JCM Accepts, published online ahead of print on 21 October 2009

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J. Clin. Microbiol. doi:10.1128/JCM.01306-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Echinocandin Minimal Inhibitory Concentrations for Candida Species in Less than Eight Hours: Comparison of the Rapid Susceptibility Assay with the Clinical and Laboratory Standards Institute's Microbroth Dilution Assay

Kevin C. Hazen^*, Dawn Dirks, and James Masuoka

Departments of Pathology and Microbiology, University of Virginia Health Center, Charlottesville, Virginia 22908-0904

^* To whom correspondence should be addressed. Email: khazen{at}virginia.edu.

Background: The echinocandins prevent fungal cell wall synthesis by inhibiting -1,3-glucan synthesis, a significant glucose consuming process. Previous studies suggested that echinocandin inhibitory activity is evident within one hour of exposure. We hypothesized that a susceptibility assay that is based on glucose consumption may provide clinically useful minimal inhibitory concentrations (MICs) rapidly.

Methods: The Rapid Susceptibility Assay (RSA), which provides MIC values in less than 8 h, was compared with the standard microbroth susceptibility assay (Clinical and Laboratory Standards Institute document M27-A3) for fifty-six Candida species strains. Variables which are known to influence MIC values with the M27-A3 method were also assessed for their effect on the RSA.

Results: Excellent agreement (>90%) between the MIC-2_RSA versus the MIC-0_M27 and the MIC-2_RSA versus the MIC-2_M27 was achieved for all three FDA approved echinocandins (micafungin, caspofungin, and anidulafungin). C. lusitaniae strains were responsible for most of the discordant results. Assay variables such as test medium, inoculum culture age, and the presence of human serum affected MIC results for the RSA and M27-A3 method similarly.

Conclusions: The RSA is equivalent to the standard M27-A3 method for determining echinocandin MICs for Candida species. The RSA provides MIC results in less than 8 h and can be applied to old and young yeast colonies. The assay could potentially provide clinically useful MICs on the same day yeast growth from a specimen is first detected on solid media.