Comparison of screening methods for detection of extended‐spectrum β‐lactamases and their prevalence among Escherichia coli and Klebsiella species in Hong Kong

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Abstract
Three tests, the disk diffusion test, the double‐disc synergy test and the inhibitor‐potentiated disc diffusion test, were compared for their abilities to detect production of extended‐spectrum β‐lactamases (ESBL) in 702 Escherichia coli and 472 Klebsiella spp. strains from four hospitals. Eleven percent E. coli and 13%Klebsiella spp. were found to produce ESBL. As an indicator of ESBL activity, the sensitivities of the five extended‐spectrum β‐lactams were as follows: cefotaxime (100%), cefpodoxime (99.3%), ceftriaxone (98.6%), aztreonam (93%) and ceftazidime (57.7%) when interpreted using the National Committee for Clinical Laboratory Standards criteria. Their positive predictive values ranged from 67.8–83.8%. Both the inhibitor‐potentiated disc diffusion test and the double‐disc synergy test (at three inter‐disc widths of 20, 25 and 30 mm) were capable of identifying all the ESBL‐producers. However, at a single inter‐disc width of 30 mm, the double‐disc synergy test has limited sensitivity (83.8%). As a second test for confirming ESBL activity in strains with reduced susceptibility to β‐lactams, the inhibitor‐potentiated disc diffusion test is therefore a simple and reliable option.