Efficacy of practised screening methods for detection of cephalosporin-resistant Enterobacteriaceae.
OBJECTIVE
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 bla(CTX-M) and bla(AmpC) 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.
Hope
R
R
Centre for Infections, Health Protection Agency, 61 Colindale Avenue, London NW9 5EQ, UK. Russell.hope@hpa.org.uk
Potz
N A C
NA
Warner
M
M
Fagan
E J
EJ
Arnold
E
E
Livermore
D M
DM
D002511:Cephalosporins / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial / Q000235:genetics; D004755:Enterobacteriaceae / Q000187:drug effects* / Q000235:genetics; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D016133:Polymerase Chain Reaction; D001618:beta-Lactamases / Q000032:analysis / Q000235:genetics
D002511:Cephalosporins; D001618:beta-Lactamases