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JAC Advance Access originally published online on November 6, 2006
Journal of Antimicrobial Chemotherapy 2007 59(1):110-113; doi:10.1093/jac/dkl431
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Efficacy of practised screening methods for detection of cephalosporin-resistant Enterobacteriaceae

R. Hope*, N. A. C. Potz, M. Warner, E. J. Fagan, E. Arnold and D. M. Livermore

Centre for Infections, Health Protection Agency 61 Colindale Avenue, London NW9 5EQ, UK

Received 3 May 2006; returned 10 July 2006; revised 5 September 2006; accepted 2 October 2006

*Corresponding author. Tel: +44-20-8327-8493; Fax +44-20-8327-6264; E-mail: Russell.hope{at}hpa.org.uk

Objectives: Enterobacteriaceae with extended-spectrum ß-lactamases (ESBLs) are now widespread and simple phenotypic tests are required to detect them in diagnostic laboratories. We investigated the performance of screening methods at 16 hospitals in South-East England.

Methods: Sixteen laboratories in South-East England submitted 1195 consecutive Enterobacteriaceae isolates found to be resistant, by their routine methods, to any or all of cefpodoxime, ceftazidime and cefotaxime. These isolates were re-tested centrally with various cephalosporin/clavulanate combinations and with multiplex PCR for blaCTX-M and blaAmpC alleles.

Results: Screening methods among the laboratories were the following: cefpodoxime discs alone (1 site); cefpodoxime, cefotaxime and ceftazidime discs (9 sites) or agar dilution (1 site); Phoenix (2 sites), Vitek 1 (1 site) and Vitek 2 (2 sites). A total of 8% of isolates submitted based on disc tests proved fully cephalosporin-susceptible, compared with 3% sent based on tests with automated systems and none of those sent based on agar dilution tests. Among isolates submitted solely on cefpodoxime resistance 256/372 (69%) proved cephalosporin-susceptible or had only borderline resistance with no clear mechanism demonstrable; this proportion decreased to 28/160 (18%) for those submitted on the basis of resistance to ceftazidime, 18/122 (15%) for those resistant to cefotaxime and 26/496 (5%) for those resistant to both cefotaxime and ceftazidime. The inference of ESBL production by Vitek 2 had the best agreement with reference laboratory results.

Conclusions: Many isolates found resistant only to cefpodoxime at the source sites proved not to have ESBLs or AmpC; screening with cefotaxime and ceftazidime allowed better specificity for identification of mechanism-based resistance, as did the automated systems. Cefpodoxime disc tests nevertheless remain a useful primary screen for laboratories prepared only to test one agent.

Keywords: ESBL detection , ß-lactamases , diagnostic tests


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