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JAC Advance Access published online on November 6, 2006
Journal of Antimicrobial Chemotherapy, 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
Received May 3, 2006
Revised September 5, 2006
Accepted October 2, 2006

Brief report

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

R. Hope 1 *, N. A. C. Potz 1, M. Warner 1, E. J. Fagan 1, E. Arnold 1, and D. M. Livermore 1

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

* To whom correspondence should be addressed.
R. Hope, E-mail: Russell.hope{at}hpa.org.uk


  Abstract

Objectives: Enterobacteriaceae with extended-spectrum {beta}-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; {beta}-lactamases; diagnostic tests.
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