Comparison of screening methods for detection of extended-spectrum beta-lactamases and their prevalence among Escherichia coli and Klebsiella speciesin Hong Kong

Three tests, the disk diffusion test, the double-disc synergy test and the inhibitor-potentiated disc diffusion test, were compared for their abilitie s to detect production of extended-spectrum beta-lactamases (ESBL) in 702 E scherichia 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 beta-lactams were as follows: cefotaxime (100%), cefpodoxime (99.3%), ceft riaxone (98.6%), aztreonam (93%) and ceftazidime (57.7%) when interpreted u sing the National Committee for Clinical Laboratory Standards criteria. The ir positive predictive values ranged from 67.8-83.8%. Both the inhibitor-po tentiated disc diffusion test and the double-disc synergy test (at three in ter-disc widths of 20, 25 and 30 mm) were capable of identifying all the ES BL-producers. However, at a single inter-disc width of 30 mm, the double-di sc synergy test has limited sensitivity (83.8%). As a second test for confi rming ESBL activity in strains with reduced susceptibility to beta-lactams, the inhibitor-potentiated disc diffusion test is therefore a simple and re liable option.