Detection of ESBL-producing <i>Klebsiella oxytoca</i> complex with VITEK 2 system and screening cutoffs for implementing confirmatory tests

<i>Klebsiella oxytoca</i> complex (<i>Ko</i>C) are important nosocomial pathogens that can be reservoirs of transmissible extended-spectrum β-lactamase (ESBL) genes. Therefore, it is essential for clinical microbiology laboratories to distinguish between <i>Ko</i>C producing ESBLs (ESBL-<i>Ko</i>C) and those hyperproducing the natural OXY-type β-lactamases (hOXY-<i>Ko</i>C). We investigated the abilities of VITEK 2 with and without using the Advanced Expert System (AES) to detect ESBL producers among 44 well-characterized <i>Ko</i>C strains (including 11 ESBL-<i>Ko</i>C and 21 hOXY-<i>Ko</i>C). VITEK 2/AES showed 100% sensitivity (Se) and 64.7% specificity (Sp), whereas the VITEK 2 coupled by the Clinical Laboratory Standards Institute (CLSI) ESBL confirmatory tests (ESBL-CTs; i.e., disk-combination tests) showed 100% Se and 97.5% Sp to detect ESBL-<i>Ko</i>C. We also analyzed <i>Ko</i>C-specific screening cutoffs for ceftriaxone (CRO), cefpodoxime (CPD), ceftazidime (CAZ), cefotaxime (CTX), and aztreonam (ATM) to negate unnecessary ESBL-CTs. As a result, we propose the following screening cutoffs (minimum inhibitory concentration [MIC] and inhibition zone diameter): CRO, >4 µg/mL and ≤16 mm; CPD, >4 µg/mL and ≤10 mm; CAZ, >1 µg/mL and ≤22 mm (European Committee on Antimicrobial Susceptibility Testing [EUCAST] disk)/≤30 mm (CLSI disk); CTX, >4 µg/mL and ≤12 mm (EUCAST disk)/≤22 mm (CLSI disk); ATM, >1 µg/mL and ≤28 mm. Notably, all suggested cutoffs could assure 100% Se and high Sp/positive predictive values for our 44 <i>Ko</i>C strains. In conclusion, the AES performed poorly, while VITEK 2 with the CLSI ESBL-CTs yielded a reliable methodology to distinguish ESBL-<i>Ko</i>C from hOXY-<i>Ko</i>C. This study also proposed revised screening cutoffs for detecting ESBL-<i>Ko</i>C and reducing the unnecessary use of ESBL-CTs.IMPORTANCESpecies within the <i>Klebsiella oxytoca</i> complex (<i>Ko</i>C) are emerging clinical pathogens of increasing concern. These bacteria can acquire plasmid-mediated ESBL genes, seriously complicating antibiotic treatment and overall management of infected patients. Differentiating ESBL-producing from non-ESBL-producing <i>Ko</i>C isolates is therefore crucial. However, this task presents significant challenges for clinical laboratories. In this work, we showed that the automated VITEK 2 system equipped with its AES fails to differentiate the two groups of <i>Ko</i>C isolates. In contrast, VITEK 2 alone followed by the ESBL screen and phenotypic confirmatory tests provides accurate differentiation. Since this latter approach increases the diagnostic workload, we also proposed new screening cutoffs for key cephalosporins that may reduce the current high number of unnecessary confirmatory tests.