Comparison of disk diffusion, VITEK 2, and broth microdilution antimicrobial susceptibility test results for unusual...

We compared the antimicrobial susceptibility testing results generated by disk diffusion and the VITEK 2 automated system with the results of the Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) reference method for 61 isolates of unusual species of Enterobacteriaceae. The isolates represented 15 genera and 26 different species, including Buttiauxella, Cedecea, Kluyvera, Leminorella, and Yokenella. Antimicrobial agents included aminoglycosides, carbapenems, cephalosporins, fluoroquinolones, penicillins, and trimethoprim-sulfamethoxazole. CLSI interpretative criteria for Enterobacteriaceae were used. Of the 12 drugs tested by BMD and disk diffusion, 10 showed >95% categorical agreement (CA). CA was lower for ampicillin (80.3%) and cefazolin (77.0%). There were 3 very major errors (all with cefazolin), 1 major error (also with cefazolin), and 26 minor errors. Of the 40 isolates (representing 12 species) that could be identified with the VITEK 2 database, 36 were identified correctly to species level, 1 was identified to genus level only, and 3 were reported as unidentified. VITEK 2 generated MIC results for 42 (68.8%) of 61 isolates, but categorical interpretations (susceptible, intermediate, and resistant) were provided for only 22. For the 17 drugs tested by both BMD and VITEK 2, essential agreement ranged from 80.9 to 100% and CA ranged from 68.2% (ampicillin) to 100%; thirteen drugs exhibited 100% CA. In summary, disk diffusion provides a reliable alternative to BMD for testing of unusual Enterobacteriaceae, some of which cannot be tested, or produce incorrect results, by automated methods.