Comparison of E-Test and Broth Microdilution Methods for Antifungal Drug Susceptibility Testing of Molds

Abstract

ABSTRACT We compared the E test with a broth microdilution method, performed according to National Committee for Clinical Laboratory Standards document M27-A guidelines, for determining the in vitro susceptibilities of 90 isolates of pathogenic molds (10 Absidia corymbifera , 10 Aspergillus flavus , 10 Aspergillus fumigatus , 10 Aspergillus niger , 10 Aspergillus terreus , 10 Exophiala dermatitidis , 10 Fusarium solani , 10 Scedosporium apiospermum , 5 Scedosporium prolificans , and 5 Scopulariopsis brevicaulis ). Overall, there was 71% agreement between the results of the two methods for amphotericin B (E-test MICs within ±2 log 2 dilutions of broth microdilution MICs) and 88% agreement with the results for itraconazole. The overall levels of agreement (within ±2 log 2 dilutions) were ≥80% for 5 of the 10 species tested against amphotericin B and 8 of the 10 species tested against itraconazole. The best agreement between the results was seen with A. fumigatus and A. terreus (100% of results for both agents within ±2 log 2 dilutions). The poorest agreement was seen with S. apiospermum , S. prolificans , and S. brevicaulis tested against amphotericin B (20% of results within ±2 log 2 dilutions). In every instance, this low level of agreement was due to isolates for which the broth microdilution MICs were low but for which the E-test MICs were much higher. The E test appears to be a suitable alternative procedure for testing the susceptibility of Aspergillus spp. and some other molds to amphotericin B or itraconazole.