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

Background

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.

Methods

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.

Results

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.

Conclusion

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.