Detection of Extended-Spectrum β-Lactamases in Klebsiellae with the Oxoid Combination Disk Method

The Oxoid combination disk method for detecting extended-spectrum β-lactamases (ESBLs) depends on comparing the inhibition zones of cefpodoxime (10-μg) and cefpodoxime-plus-clavulanate (10- plus 1-μg) disks. The presence of clavulanate enlarged the zones for all of 180 ESBL-producing klebsiellae by ≥5 mm, whereas zones for cefpodoxime-susceptible isolates and cefpodoxime-resistant isolates with AmpC and K1 β-lactamases were enlarged by ≤1 mm. Good discrimination was achieved with either the NCCLS or the British disk method.