Objective: The Clinical and Laboratory Standards Institute (CLSI) M38-A2 broth microdilution (BMD) method for antifungal susceptibility testing of filamentous fungi now includes guidelines for testing echinocandin activity using the minimum effective concentration (MEC) as the endpoint measurement. The objective of this study was to determine if the caspofungin Etest could accurately and reliably measure the MEC of a large collection of clinical Aspergillus isolates. Methods: Caspofungin activity against Aspergillus isolates was tested by BMD (CLSI M38-A2) for MEC (visual and microscopic) determination and by Etest for MIC using both a RPMI agar and Mueller-Hinton agar (supplemented with glucose and methylene blue [MGM]). Etest was performed using a modification of the CLSI M44-A disk diffusion method and the inocula prepared for BMD. All tests were incubated for 24 h. Etest MICs were recorded as the lowest concentration of caspofungin where the zone of growth inhibition intersected with the Etest strip. MICs were rounded up to the next even log2 concentration for the evaluation. Etest values + 1 log2 dilution of the reference BMD MEC were considered to be in essential agreement (EA). Results: BMD MEC was determined for 345 clinical Aspergillus isolates, including 105 A. fumigatus, 62 A. flavus, 104 A. niger, 50 A. nidulans, and 24 A. terreus. Regardless of the Aspergillus species or agar medium tested, the caspofungin Etest consistently produced a markedly lower growth inhibition endpoint (MIC). The EA of the Etest on MGM and RPMI to BMD MEC was 18% and 26%, respectively. The geometric mean values for BMD MEC vs MGM Etest were 0.137 Âµg/ml and 0.024 Âµg/ml, respectively, and the geometric mean values for BMD vs RPMI were 0.128 Âµg/ml and 0.031 Âµg/ml, respectively. Comparatively, 91% of paired MGM and RPMI Etest results were within 2 log2 dilutions of each other and consistently produced clearly defined endpoints. Conclusion: BMD for filamentous fungi is not an ideal test format for many clinical laboratories and the simplicity of the Etest may make it a more viable option. Although the results from this study demonstrate a clear discordance between the absolute values generated by the caspofungin BMD and Etest, it is difficult to dismiss the consistent performance of the Etest. An expanded optimization of test conditions will hopefully improve Etest concordance with BMD MEC without compromising consistency.
Full conference title:
20th European Congress of Clinical Microbiology and Infectious Diseases
- ECCMID 20th (2010)