Laboratory detection of extended-spectrum-beta-lactamase-producing Enterobacteriaceae: evaluation of two screening agar plates and two confirmation techniques.

The worldwide prevalence of extended-spectrum-beta-lactamase-producing ESBL-producing Enterobacteriaceae (ESBL-E) is increasing, making the need for optimized detection techniques more urgent. In this study we investigated the performance of two ESBL-E screening and two ESBL-E confirmation techniques. In accordance with the Dutch national guidelines (www.wip.nl), a collection of 642 highly resistant Enterobacteriaceae strains, as identified by Vitek2, was used to test the performances of two screening techniques (EbSA ESBL agar plate and ChromID ESBL agar plate) and of two confirmation techniques (MIC-strip ESBL and Vitek2 ESBL test panel). The individual test results were compared by using Etest, followed by a combination disk test if Etest results were inconclusive. Among group 1 isolates (Escherichia coli, Klebsiella spp., Proteus spp., Salmonella spp., and Shigella spp.) 291 (57.6%) were ESBL-E, versus 65 (47.4%) in group 2 (Enterobacter spp., Citrobacter spp., Morganella morganii, Serratia spp., and Providencia spp.). The sensitivities of all four tests for group 1 were comparable (EbSA, 96.6%; ChromID, 97.3%; MIC-strip, 99.6%; and Vitek2, 95.1%). The specificities of the EbSA and ChromID were the same (93.9%). However, the confirmation techniques produced many inconclusive test results, which reduces the applicability in routine laboratories. Only the two screening agar plates were validated for ESBL testing of group 2 microorganisms. They showed comparable sensitivities; however, the EbSA screening agar plate had a significantly higher specificity (78.6% versus 44.3%). In conclusion the screening agar plates performed better than the two confirmation techniques. The EbSA agar plate had the best overall performance.