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Antifungal susceptibility testing using the E test: comparison with the broth macrodilution technique
S C Chen1, M L O'Donnell, S Gordon
1Department of Clinical Microbiology, Westmead Hospital, New South Wales, Australia.
The Journal of Antimicrobial Chemotherapy
|February 1, 1996
Summary
The E test shows promise as an alternative antifungal susceptibility testing method. However, fluconazole MICs for Cryptococcus neoformans require cautious interpretation due to potential inaccuracies.
Area of Science:
- Medical Mycology
- Clinical Microbiology
- Antifungal Drug Discovery
Background:
- Accurate antifungal susceptibility testing is crucial for guiding treatment decisions.
- The E test is a commercially available method for determining minimum inhibitory concentrations (MICs).
- Standardized methods like broth macrodilution are essential for reliable susceptibility data.
Purpose of the Study:
- To compare the E test with the National Committee for Clinical Laboratory Standards (NCCLS) reference broth macrodilution method.
- To evaluate the agreement of MICs obtained by both methods for clinical isolates of Candida spp. and Cryptococcus neoformans.
- To assess the E test's potential as an alternative antifungal susceptibility testing method.
Main Methods:
- Testing of 86 clinical isolates, including Candida spp. and Cryptococcus neoformans.
- Utilizing the NCCLS reference broth macrodilution method as a standard.
- Employing the E test for comparative MIC determination.
Main Results:
- MIC agreement rates between the E test and broth macrodilution for Candida spp. ranged from 73-89% within +/-1 doubling dilution and 87-100% within +/-2 dilutions.
- For Cryptococcus neoformans, agreement within +/-1 dilution exceeded 70% for most antifungal agents.
- A notable exception was fluconazole against C. neoformans, with only 35% agreement, indicating potential for falsely elevated MICs.
Conclusions:
- The E test demonstrates potential as a viable alternative method for antifungal susceptibility testing.
- Further evaluation is warranted, especially for specific pathogens and antifungal combinations.
- E test MICs for fluconazole against Cryptococcus neoformans, particularly C. neoformans var. gattii, should be interpreted with caution due to potential MIC elevation.