Evaluation of two commercially available methods used for the rapid detection of ESBL-producing strains

Detection of ESBL production by Enterobacteriaceae remains a challenge for microbiologists. Although recent changes in the breakpoints of third-generation cephalosporins decreased the likelihood of reporting ESBL producers as susceptible to these compounds, ESBL detection is of interest for prevention of dissemination of ESBL-producers by cross-transmission and for epidemiological purpose. The aim of this study was to evaluate the sensitivity and the specificity of two commercial methods suitable for rapid ESBL-detection in Gram negative bacilli: the ChromID ESBL medium (bioMérieux) and the Cica-ß-Test (Mast Group, Merseyde, UK). 121 Enterobacteriaceae collected between February 2008 and April 2009 at the Laboratory Analysis IRCCS Humanitas were tested for ESBL-production by PhoenixTM, E-test (ESBL reference test), ChromID ESBL medium and Cica-β-Test. ChromID showed 100% of sensitivity and specificity for the screening of ESBL in E. coli; lower values of specificity were found in the case of P. mirabilis (81%) and Klebsiella spp. (92%). The Cica-β-Test always showed high specificity levels, but the poor sensitivity found for both E. coli (90%), P. mirabilis (73%) and Klebsiella spp. (85%), discourages its use for screening of ESBL in Gram negative bacilli from blood-cultures. Rapid identification of ESBL producers is of interest to implement hygiene precautions. In that case, using a very sensitive primary test is of major interest.