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Evaluation of three different methodologies for the detection of extended spectrum beta-lactamases



Evaluation of three different methodologies for the detection of extended spectrum beta-lactamases



Abstracts of the General Meeting of the American Society for Microbiology 102: 118



Accurate detection of clinically relevant resistance mechanisms is a major responsibility of the clinical microbiology laboratory. Extended-spectrum beta-lactamases (ESBL) are difficult to detect due to the large number of ESBLs described, the variable spectrum of resistance to beta-lactam agents, and MIC levels that are interpreted as susceptible following National Committee for Clinical Laboratory Standards (NCCLS) guidelines. In addition, other resistance mechanisms may be present in combination with or instead of ESBL production. Determination of the resistance mechanism present is a major factor in the selection of appropriate therapy by the clinician and the reporting of susceptibility results. If the laboratory does not accurately detect ESBL production, inappropriate therapy leading to treatment failure, increased healthcare costs, longer morbidity and hospital stays may result. We evaluated the ability of three different methodologies to detect ESBL production in Escherichia coli, Klebsiella oxytoca, and K. pneumoniae. In addition, isoelectric focusing (IEF) was performed to provide insight into the different beta-lactamases produced by each isolate. The methods evaluated included disk diffusion following NCCLS guidelines; a Dade MicroscanTM ESBL confirmation panel; and the bioMerieux Vitek 2TM susceptibility card with review by the Advanced Expert System (AES). Organisms were specifically selected for evaluation if they demonstrated that ESBL production was not easily detected by NCCLS guidelines (i.e., zone sizes near the NCCLS ESBL detection breakpoints or discrepancies between different methodologies). A total of 20 isolates were tested (11-E. coli, 8-K. oxytoca, and 1-K. pneumoniae). The MicroscanTM ESBL panel reported all 20 isolates as ESBL producers, the VitekTM AES system reported 12 as potential ESBL producers, and disk diffusion reported 9 isolates as ESBL producers. Seventeen of twenty isolates had an ESBL identified by IEF. In conclusion, different in vitro methodologies give different results. Until a consensus answer is obtained, NCCLS guidelines should be followed by clinical laboratories.

Accession: 034886486

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