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Journal of Clinical Microbiology, February 2000, p. 656-661, Vol. 38, No. 2
Department of Pathology, University of Texas
Medical Branch, Galveston, Texas 77555-07401;
Microbiology Service, Clinical Pathology Department, W. G. Magnuson Clinical Center, National Institutes of Health, Bethesda,
Maryland 208922; Department of
Pathology, University of Texas-Houston Medical School, Houston,
Texas 770303; StatProbe, Ann Arbor,
Michigan 481084; and Department of
Microbiology, University of Texas Health Center at Tyler, Tyler, Texas
757105
Received 8 September 1999/Returned for modification 27 October
1999/Accepted 19 November 1999
A multicenter study was conducted to assess the inter- and
intralaboratory reproducibility of the Etest for susceptibility testing
of the rapidly growing mycobacteria. The accuracy also was evaluated by
comparing Etest results to those obtained by broth microdilution. Ten
isolates (four of the Mycobacterium fortuitum group, three
of Mycobacterium abscessus, and three of
Mycobacterium chelonae) were tested against amikacin,
cefoxitin, ciprofloxacin, clarithromycin, doxycycline, imipenem, and
trimethoprim-sulfamethoxazole in each of four laboratories. At each
site, isolates were tested three times on each of three separate days
(nine testing events per isolate) using common lots of media and Etest
strips. Interlaboratory agreement among MICs (i.e., mode ± 1 twofold dilution) varied for the different drug-isolate combinations
and overall was best for trimethoprim-sulfamethoxazole (75% for one
isolate and 100% for all others), followed by doxycycline and
ciprofloxacin. Interlaboratory agreement based on interpretive category
also varied and overall was best for doxycycline (100% for all
isolates), followed by trimethoprim-sulfamethoxazole and ciprofloxacin.
Interlaboratory reproducibility among MICs was most variable for
imipenem, and agreement by interpretive category was lowest for
imipenem and amikacin. Modal Etest MICs agreed with those by broth
microdilution only for doxycycline and the sulfonamides. For all other
drugs, the modal MICs by the two methods differed by more than ± 1 twofold dilution for one or more isolates. In all cases, the Etest
MIC was higher and would have caused reports of false resistance. In
summary, the Etest in this evaluation did not perform as well as broth
microdilution for susceptibility testing of the rapidly growing
mycobacteria. It was problematic for most species and drugs, primarily
because of a trailing endpoint and/or high MICs compared to broth. Its
use will necessitate further investigation, including determination of
the optimal medium and incubation conditions and clarification of
endpoint interpretation.
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Multisite Reproducibility of Etest for
Susceptibility Testing of Mycobacterium abscessus,
Mycobacterium chelonae, and Mycobacterium
fortuitum
*
Corresponding author. Mailing address: Department of
Pathology, University of Texas Medical Branch, Galveston, TX
77555-0740. Phone: (409) 772-4851. Fax: (409) 772-5683. E-mail:
gwoods{at}utmb.edu.
Journal of Clinical Microbiology, February 2000, p. 656-661, Vol. 38, No. 2
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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