The accuracy of extended-spectrum beta-lactamase detection in Escherichia coli and Klebsiella pneumoniae in South African laboratories using the Vitek 2 Gram-negative susceptibility card AST-N255

Background:  Phenotypic detection of extended-spectrum beta-lactamases (ESBLs) is based on the inhibition of ESBL enzymes by β-lactamase inhibitors and on the comparison of cephalosporin activity with or without a β-lactamase inhibitor. Many South African diagnostic laboratories rely on the Vitek 2 for automated susceptibility testing and for ESBL detection. However, the Gram-negative susceptibility card currently used locally (AST-N255) has been modified and its accuracy for ESBL detection is not known. Methods:  We randomly selected 50 isolates of  Klebsiella pneumoniae  and  Escherichia coli  from a collection of clinical bloodstream isolates from Groote Schuur Hospital from 2015 to 2016, including ESBL-producing and non-ESBL-producing strains. We used standardised phenotypic (disc diffusion and broth microdilution) and genotypic (conventional polymerase chain reaction (PCR) for  bla CTX-M , bla SHV  and  bla TEM ) methods for detection of ESBLs. We compared ESBL detection by Vitek 2 to a composite reference standard comprising ESBL detection either by both phenotypic methods or by one phenotypic method together with genotypic detection. Results:  The sensitivity of Vitek 2 system for detection of ESBLs was 33/36 or 92% (78% – 97%) for  E. coli , and 40/40 or 100% (91% – 100%) for  K. pneumoniae , whilst specificity was 10/10 or 100% (72% – 100%) and 9/10 or 90% (60% – 98%), respectively. This is comparable with previous studies. Conclusion:  Using a composite reference standard of the phenotypic and genotypic methods employed in this study, no Vitek-categorised ESBL  E. coli  or  K. pneumoniae  was found to be a non-ESBL with the exception of possible misinterpretation with  K. pneumoniae  SHV-hyper-producing isolates.