Evaluation of the current National Committee for Clinical Laboratory Standards guidelines for screening and confirming extended-spectrum beta-lactamase production in isolates of Escherichia coli and Klebsiella species from bacteremic patients

The National Committee for Clinical Laboratory Standards (NCCLS) recommendations for screening and confirming the production of extended-spectrum beta-lactamases (ESBLs) were evaluated in 115 isolates of Escherichia coli and 157 isolates of Klebsiella spp. from Israeli patients with bacteremia. All isolates were screened using cefotaxime, ceftazidime, and cefpodoxime discs. Confirmatory tests using pairs of discs containing ceftazidime, cefotaxime, or cefpodoxime in which clavulanic acid was added to one of the discs in each pair [inhibitor-potentiated disc diffusion test (IPDDT)] and two double-sided E test strips containing ceftazidime or cefotaxime with and without clavulanic acid were performed on all isolates regardless of the results of screening tests. Isolates that tested positive by one or more confirmatory tests were considered ESBL producers. Overall, 69 (25.4%) strains were found to be ESBL producers. The sensitivity of the NCCLS screening criteria ranged between 98.6% for cefotaxime and 92.8% for ceftazidime, and the specificity ranged between 100% for cefotaxime and cefpodoxime and 99.0% for ceftazidime. The sensitivity of the confirmatory tests ranged between 97.1% for the cefotaxime E test and only 75.4% for the ceftazidime IPDDT discs. All 64 isolates that fell in the intermediate and resistant categories for cefotaxime, as well as all 41 in the same categories for ceftazidime and 68 of 69 in these categories for cefpodoxime, were confirmed as ESBL producers. The use of multiple antimicrobial discs for screening isolates and combinations of IPPDT discs is needed to improve the sensitivity of confirmatory testing. It is recommended that isolates falling in the intermediate and resistant categories in disc diffusion testing be reported as ESBL producers. The use of confirmatory tests should be limited to organisms with inhibition zone diameters ranging between the NCCLS recommendations for ESBL screening and the intermediate category breakpoints.