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Caspofungin Etest endpoint for Aspergillus isolates shows poor agreement with the reference minimum effective
Jeff Fuller1, Adam Schofield, Safeer Jiwa
1Division of Medical Microbiology, Department of Laboratory Medicine and Pathology, University of Alberta Hospital, Walter C. Mackenzie Centre 2B3.13, 8440 112 St., Edmonton, Alberta T6G 2J2, Canada. jeff.fuller@albertahealthservices.ca
Comparing caspofungin Etest MIC to the reference broth microdilution minimum effective concentration (MEC) for Aspergillus isolates showed Etest results were significantly lower. Further optimization is needed to avoid underreporting echinocandin resistance.
Area of Science:
- Medical Mycology
- Antimicrobial Susceptibility Testing
- Clinical Microbiology
Background:
- The Clinical and Laboratory Standards Institute (CLSI) M38-A2 method guides antifungal susceptibility testing for filamentous fungi.
- Echinocandin antifungal activity is now assessed using the minimum effective concentration (MEC) endpoint.
Purpose of the Study:
- To compare caspofungin Etest MICs with the CLSI M38-A2 BMD MEC for clinical Aspergillus isolates.
- To evaluate Etest performance on RPMI agar and Mueller-Hinton agar supplemented with glucose and methylene blue (MGM).
Main Methods:
- Caspofungin Etest MICs were determined on RPMI and MGM agars for 345 clinical Aspergillus isolates.
- Results were compared to the reference broth microdilution (BMD) MEC values.
- Essential agreement and geometric mean values were calculated.
Main Results:
- Essential agreement between Etest on MGM/RPMI and BMD MEC was low (18% and 26%).
- Etest MICs were markedly lower than BMD MECs (geometric means 0.024-0.031 µg/mL vs. 0.128-0.137 µg/mL).
- Etest provided reproducible endpoints, with 91% of paired MGM and RPMI results within 2 log(2) dilutions.
Conclusions:
- Caspofungin Etest MIC is a reproducible endpoint but consistently underestimates the MEC compared to BMD.
- Optimization studies are necessary to improve concordance between Etest and BMD assays.
- Failure to optimize may lead to underreporting of echinocandin resistance in Aspergillus species.
