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Critical Care

Volume 12 Supplement 5

Sepsis 2008

Open Access

Evaluation of an agar-gradient minimum-inhibitory-concentration method (the Etest) as a rapid and direct measure of antimicrobial susceptibility in Gram-negative bacteraemia

  • Tom Darton1,
  • David Partridge1,
  • Steve Davis1 and
  • Rob Townsend1
Critical Care200812(Suppl 5):P25

Published: 18 November 2008


MeropenemAntimicrobial SusceptibilityCefoxitinampCTazobactam


The selection of appropriate antibiotics to treat Gram-negative bacteraemia may be life-saving. Rapid methods of antimicrobial susceptibility testing have sought to guide early antibiotic selection and usage. We sought to evaluate whether a combination of chromogenic agar and six Etest gradient diffusion strips could be used to provide a clinically useful, direct rapid antimicrobial susceptibility test result following 4 hours of incubation.


Fifty consecutive Gram-negative blood culture isolates were tested over a 4-month period. Following confirmation of Gram-negative bacilli, 200 μl blood was directly inoculated from the enrichment broth onto a chromogenic MH agar plate. Six Etest strips were directly applied onto the agar using an automated placement device (Simplex C76; Inverness Medical UK (Bio-Stat Division), Stockport, UK). The antibiotics used were cefoxitin, cefotaxime and ceftazidime (to indicate the presence of extended-spectrum and ampC β-lactamase producers), vancomycin (to 'screen' for Gram-positive organisms), and piperacillin–tazobactam and meropenem (based on local prescribing patterns). The plates were incubated at 35 to 37°C and read at 4, 6 and 24 hours. Minimum-inhibitory concentration values were determined and organisms were categorized as susceptible/resistant according to British Society for Antimicrobial Chemotherapy breakpoints. A presumptive identification and susceptibility profile was obtained at 4 hours, based upon which the investigators recorded a decision as to whether the patients' antibiotics could be escalated, de-escalated or remain unchanged. The results were correlated with those at 6 and 24 hours, and with the report issued following routine susceptibility testing, as performed at our institution.


Forty-five (90%) cultures had Etest susceptibility profiles interpretable at 4 hours. Of the five remaining, three (6%) were read at 6 hours and two (4%) at 24 hours. In three cases there was a mixed growth of organisms. Twelve (24%) organisms had resistance mechanisms identified, of which 10 (83%) were confirmed by our routine antimicrobial susceptibility rest method. At 4 hours, nine (18%) patients were receiving too narrow spectrum antibiotics. Additionally, the investigators felt that antibiotics could have been safely de-escalated in 16 (32%) cases and continued in the remaining 25 (50%) patients.


Our evaluation of this method has shown that it can provide rapid and reliable antibiotic susceptibility information at 4 hours. This could potentially have a major impact on antibiotic use and may significantly affect patient management.

Authors’ Affiliations

University of Sheffield, School of Biomedical Sciences and Sheffield Teaching Hospitals NHS Foundation Trust, Department of Medical Microbiology, Sheffield, UK


© Darton et al; licensee BioMed Central Ltd. 2008

This article is published under license to BioMed Central Ltd.