Journal of Antimicrobial Chemotherapy, Vol 42, 321-331, Copyright © 1998 by The British Society for Antimicrobial Chemotherapy
DW Warnock, EM Johnson and TR Rogers
Ten laboratories tested 18 isolates of Candida spp. and two of Cryptococcus
neoformans against amphotericin B, flucytosine, fluconazole and
itraconazole on two occasions by the Etest method. Two individuals read
each set of results. Of the 18 isolates of Candida spp., five were
duplicated, but the participants were not told this. In 40 of the 60
drug-organism combinations studied, at least 80% of the Etest MICs fell
within a five-concentration range corresponding to the modal MIC +/- 1 log2
dilution. In five combinations, >50% of the Etest MICs fell outside this
five-concentration range. In 17 (85%) of the 20 drug-organism combinations
tested in duplicate, at least 80% of the paired Etest results fell within
two concentrations of each other (corresponding to one log2 dilution).
Overall, 88.5% of the paired Etest results for amphotericin B agreed to
within two concentrations, as did 94% of results for flucytosine, 92% for
fluconazole and 79% for itraconazole. The broth microdilution MICs of the
four antifungal agents for the 15 isolates were measured on five occasions
in the Mycology Reference Laboratory, Bristol. In each case, the results
fell within a three log2 concentration range. In 24 (40%) of the 60 drug-
organism combinations tested, at least 80% of the Etest results fell within
the broth microdilution test MIC range, but 27 (45%) showed <50% exact
agreement. In 33 (73%) of 45 drug-organism combinations involving
flucytosine, fluconazole or itraconazole, at least 80% of the Etest results
fell within the same class (susceptible, resistant, or susceptible
dependent upon dose) as the broth microdilution results. With fluconazole,
the Etest method misclassified three susceptible isolates of Candida spp.
as resistant in 1.5-15% of tests. With itraconazole, the Etest
misclassified seven susceptible isolates of Candida spp. as resistant in
5-62.5% of tests. The Etest also misclassified both C. neoformans isolates
as resistant to flucytosine, fluconazole and itraconazole in 7.5-65% of
tests. Our results suggest that the Etest is suitable for routine use with
Candida spp. and amphotericin B or flucytosine. It is less reliable for the
azoles, and isolates that appear to demonstrate acquired resistance should
be retested with well-established reference methods.
REVIEWS
Multi-centre evaluation of the Etest method for antifungal drug susceptibility testing of Candida spp. and Cryptococcus neoformans. BSAC Working Party on Antifungal Chemotherapy
PHLS Mycology Reference Laboratory, Public Health Laboratory, Bristol, UK.
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