Caspofungin Etest endpoint for Aspergillus shows poor essential agreement with the reference minimum effective concentration

Abstract number: P1359

Fuller J., Schofield A., Sand C., Brietkreutz L., Turnbull L., Rennie R.

Objective: The minimum effective concentration (MEC) is the currently accepted broth microdilution (BMD) endpoint for measuring echinocandin activity against filamentous fungi. However, this method requires microscopic examination of fungal growth, which prevents many laboratories from adopting this methodology. We evaluated the caspofungin Etest as an alternative to the BMD for MEC determination in Aspergillus.

Methods: BMD was performed according to the Clinical Laboratories Standards Institute (CLSI) document M38-A using 0.016 to 32 mg/L caspofungin. MEC was determined microscopically for the first 30 specimens, correlated to the macroscopic appearance of each well, and then inferred from macroscopic appearance for remaining isolates. Etest was performed using a modification of the CLSI M44-A disk diffusion method on Mueller-Hinton agar supplemented with glucose (2%) and methylene blue (0.5 mg/L). Caspofungin Etest endpoints were measured at the intersection between Aspergillus growth and the Etest strip. Representative CLSI-approved quality control strains were tested to satisfy reproducibility. All tests were incubated for 24 hours.

Results: 274 clinical Aspergillus isolates were tested, including A. fumigatus (73), A. flavus (51), A. niger (90), A. terreus (19), and A. nidulans (41). The Etest endpoint was lower than BMD for 91% of clinical isolates with an overall essential agreement (leqslant R: less-than-or-eq, slant1 doubling dilution variance) of 18.25%. The majority of discordant Etest results varied from BMD by 2 (26%), 3 (33%), and 4 (15%) doubling dilutions, and was proportional across all species. Etest endpoints were clearly defined except for A. flavus and A. terreus, which tended to yield narrower inhibition zones with microcolony growth (disregarded). This discordance between BMD and Etest was reproducible using CLSI-approved quality control strains daily for a 10-day period.

Conclusions: Although susceptibility testing for filamentous fungi to the echinocandins is still in its infancy, the results of this study demonstrate that the caspofungin BMD MEC and Etest endpoint are not comparable measures. Unless CLSI test conditions can be optimised to reduce this disparity the caspofungin Etest cannot be used as a BMD alternative for Aspergillus susceptibility testing.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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