Evaluation of the VITEK®2 AST N-054 test card for the detection of extended-spectrum β-lactamase production in Escherichia coli with CTX-M phenotypes

Objectives: A new VITEK®2 antibiotic susceptibility testing (AST) card, AST N-054, was introduced for aerobic Gram-negative bacilli in 2007 and has been widely adopted for routine use in the UK. We evaluated its performance for detecting extended-spectrum β-lactamase (ESBL) production in Escherichia coli. Methods: ESBL-producing faecal isolates of E. coli (n = 137) from residents in nursing homes were tested using the AST N-054 card on VITEK®2 and with MASTDISCS® ID ESBL detection disc diffusion tests (Mast Diagnostics, Bootle, UK). The susceptibility result recommended by the VITEK®2 software was also recorded. Results: The AST N-054 card detected ESBL production in 93 of the 137 isolates tested [test sensitivity 67.9% (95% CI, 59.7-75.1)]. E. coli strain A, a widespread lineage in the UK with a low-level CTX-M enzyme production, accounted for most of the detection failures, with 35/73 strain A isolates incorrectly reported versus 9/64 non-strain A isolates (P < 0.0001). The MASTDISCS® correctly detected ESBL in 135/137 isolates [test sensitivity 98.5% (95% CI, 94.5-99.9)]. Of the 44 isolates found to be negative for ESBL production by VITEK®2, the Advanced Expert System misreported 29 as susceptible to cefotaxime and all as susceptible to ceftazidime and aztreonam. Conclusions: These data suggest that the AST N-054 card for the VITEK®2 system is less reliable than other previously reported cards for the detection of CTX-M β-lactamase-producing E. coli circulating in the UK, particularly strain A isolates.