Comment on: Performance of the Oxoid M.I.C.Evaluator™ Strips compared with the Etest® assay and BSAC agar dilution.

Sir, We read with great interest the study published by Mushtaq et al. about the performance of Oxoid M.I.C.EvaluatorTM (M.I.C.E.) (Thermo Fisher Scientific, Basingstoke, UK) strips compared with those of the Etest (AB bioMérieux, Marcy l’Étoile, France) and agar dilution for determination of MICs using the BSAC guidelines. We recently had the opportunity to carry out a similar study that comparatively evaluated the performance of this new gradient strip with those of the Etest and broth microdilution (BMD) according to CLSI (formerly NCCLS) guidelines. A total of 160 clinical bacterial isolates were randomly selected and 38 antimicrobial/organism combinations were tested by each method as follows (see Table 1): Pseudomonas aeruginosa (n1⁄420) and Acinetobacter spp. (n1⁄420) (non-fermenters group; gentamicin, ciprofloxacin and imipenem); Klebsiella pneumoniae (n1⁄420) and Escherichia coli (n1⁄420) (Enterobacteriaceae group; ampicillin, amoxicillin/clavulanic acid, gentamicin, ciprofloxacin, cefotaxime and imipenem); Staphylococcus aureus (n1⁄420) and coagulasenegative Staphylococcus (n1⁄420) (staphylococci group; ciprofloxacin, erythromycin, gentamicin, oxacillin, linezolid and vancomycin); and Enterococcus faecalis (n1⁄420) and Enterococcus faecium (n1⁄420) [enterococci group; ampicillin, gentamicin high level (HL), linezolid and vancomycin]. Only a single isolate per patient was evaluated. E. coli ATCCTM 25922, P. aeruginosa ATCCTM 27853, S. aureus ATCCTM 29213 and E. faecium ATCCTM 29212 were tested as quality control strains and MIC results were within the expected ranges. The MICs were read independently by three observers. In general, the essential agreement (MIC within +1 log2 dilution) rate between M.I.C.E. and the BMD reference method was very good (94.2%), and was similar to that obtained by Etest versus BMD (96.4%). These rates were higher than those observed by Mushtaq et al. (.89% for M.I.C.E. and .87% for Etest). In our study, comparison of BMD MIC results with those of M.I.C.E. showed ≥90% essential agreement for ampicillin, amoxicillin/clavulanic acid, ciprofloxacin (except for staphylococci, 85%), erythromycin, gentamicin, gentamicin HL, imipenem, linezolid and oxacillin (Table 1). Although cefotaxime (85%), ciprofloxacin (85% for staphylococci) and vancomycin (72.5% and 80% for staphylococci and enterococci, respectively) demonstrated lower essential agreement rates, they all yielded minor, major and very major errors, respectively, within the acceptable percentages according to CLSI guidelines. Very major errors were not observed in our study. Mushtaq et al. reported similar essential agreement rates for cefotaxime (89.9%) tested against Enterobacteriaceae, but not for the vancomycin/Gram-positive cocci combinations. This difference could be explained by the distinct reference method employed in our study (BMD). Mushtaq et al. reported a poor correlation with Enterobacteriaceae versus imipenem strips (87.7% and 82.5% for M.I.C.E. and Etest, respectively). In contrast, our results showed excellent performance of the imipenem gradient strips (97.5% and 100.0% for M.I.C.E. and Etest, respectively) against Enterobacteriaceae. These conflicting results are caused by the proportions of different Enterobacteriaceae tested, particularly Proteus spp., which gave the biggest discrepancy in the study by Mushtaq et al. Categorical agreement rates between BMD and M.I.C.E. were .92% for all antimicrobials tested except for amoxicillin/clavulanic acid (62.5%). Amoxicillin/clavulanic acid MICs by M.I.C.E. strips for Enterobacteriaceae tended to be 1 log2 dilution higher than broth microdilution MIC values. This poor agreement with amoxicillin/clavulanic acid was also reported by Mushtaq et al. (78.3% essential agreement with Enterobacteriaceae). Our findings should be confirmed by testing a higher number of Enterobacteriaceae isolates. Comparison of M.I.C.E. MIC results with those obtained by Etest showed 100% essential agreement for erythromycin, gentamicin HL, imipenem, vancomycin and oxacillin. In general, essential agreement rates were ≥96% for all antimicrobials tested except for amoxicillin/clavulanic acid (67.5%). As new commercial methods for susceptibility testing become available, it is important to establish the accuracy of such methodologies compared with reference methods, BMD and/or agar dilution. This study demonstrated that M.I.C.E. MICs were comparable to those obtained by BMD (≥90%) and Etest (≥96%) for most antimicrobial/organism combinations evaluated using CLSI recommendations. Our study indicates that M.I.C.E. methodology may constitute an acceptable alternative for susceptibility testing of commonly isolated pathogens by routine clinical microbiology laboratories.