Evaluation of the VITEK 2 Advanced Expert System performance for predicting resistance mechanisms in Enterobacterales acquired from a hospital-based screening program.

There is limited literature examining the accuracy of the VITEK 2 Advanced Expert System (AES) in characterisation of β-lactamase resistance patterns. We present a prospective single centre study to better ascertain the performance characteristics of this program. The VITEK 2 AES interpretation was compared to established laboratory phenotypic methods. The overall sensitivity for detection of broad-spectrum β-lactamase by the AES was 95%, with a specificity of 78%. One or more discrepancies were noted in 36% of samples, with the majority of these (87/100) due to incorrect 'overcall' of a resistance mechanism. AES characterisation of AmpC resistance mechanisms was excellent. In contrast, the AES had poor specificity in classifying extended spectrum β-lactamases (ESBLs). As a screening aid, the AES can be a valuable tool. However, optimal use requires an adequate working knowledge of resistance mechanisms in order to correctly interpret and accept the result output.