Evaluation of Oxoid combination discs for detection of extended-spectrum β-lactamases

Objectives: We evaluated the reliability of cefpirome/clavulanate (CD04) compared with ceftazidime/clavulanate (CD02) and cefotaxime/clavulanate (CD03) Oxoid combination discs for the detection of extended-spectrum beta-lactamases (ESBL) in several Enterobacteriaceae isolates, including Enterobacter spp. Methods: Overall, a total of 105 ESBL-positive [positive double-disc synergy test (DDST)] and 94 ESBL-negative (negative DDST) Gram-negative isolates were evaluated. Ninety-eight isolates were confirmed as ESBL-positive on the basis of the sequence alignments of the bla(TEM) and/or bla(SHV) gene amplification products, which matched with previously identified ESBLs. The phenotypic detection of ESBLs was performed by the three combination discs according to the NCCLS and BSAC methods. The CD04 disc was evaluated with the manufacturer's recommended zone size difference breakpoint of greater than or equal to4 mm. Results: In Escherichia coli and Klebsiella spp., the sensitivities (%)/specificities (%) of CD02, CD03 and CD04 discs, and the combination of CD02 or CD04 discs, were, respectively, 88/92, 90/92, 95/84 and 100/82, while the corresponding figures were 94/100, 4/100, 94/100 and 100/100 in Enterobacter aerogenes. NCCLS and BSAC methods yielded concordant results in 99% of the isolates. Conclusions: CD04 and CD02 discs were the best combination for detection of ESBLs in our collection of Enterobacteriaceae isolates, including E. aerogenes.