Evaluation of Etest for susceptibility testing of rapidly growing mycobacteria

MICs of amikacin, cefoxitin, ciprofloxacin, clarithromycin, doxycycline, and imipenem were determined by Etest for 100 clinical strains of rapidly growing mycobacteria and compared with MICs determined by a reference agar dilution method. Etest MICs were also determined by an alternative inoculum application (agar overlay) method and compared with MICs determined by the inoculum application method recommended by the manufacturer (swabbing). Agreement between Etest and agar dilution MICs within +/- 1 log2 dilution was 85% (511 of 600), and agreement within +/- 2 log2 dilutions was 97% (580 of 600). The rate of complete category agreement was 88%, and rates of major and minor errors were 2.2 and 11.7%, respectively. No very major errors were detected for Etest MICs. Interlaboratory agreement between MICs determined at two separate laboratories was 81% (121 of 149) within +/- 1 log2 dilution and 92% (137 of 149) within +/- 2 log2 dilutions. Agreement between laboratories by interpretive category was 92%. Exact agreement between agar overlay and swab application MICs was 52.3%, and agreement within +/- 1 log2 dilution was 82.3%. Diffuse ellipse edges and trailing growth were still a problem with the overlay method, and in some cases results were more difficult to interpret than they were with the corresponding swab-prepared plate. In summary, our data suggest that Etest may be an accurate and reproducible method for determining susceptibility of rapidly growing mycobacteria.