Discrepancy between genotypic and phenotypic ESBL rates in E. coli from intra-abdominal infections in the United States.

This report describes discrepancies in <italic>E. coli</i> genotypic and phenotypic ESBL rates observed in a surveillance study of 29 United States hospitals that participated in the Study for Monitoring Antimicrobial Resistance Trends (SMART). The ESBL phenotype was determined with the CLSI confirmatory broth microdilution test using cefotaxime and ceftazidime with and without clavulanate. Genes encoding ESBLs, carbapenemases, and plasmidic AmpC β-lactamases were detected using a combination of microarray and multiplex PCR assays. Among 168 molecularly characterized phenotypically ESBL-positive <italic>E. coli</i> isolates from intra-abdominal infections, 4.8% were genotypically negative from 2009-2012 and 29.5% in 2013. Due to the high rate of false-positive phenotypic ESBL results in 2013, the 5-year phenotypic ESBL trend was skewed and showed a statistically significant increase (p<0.05) in ESBL-positive <italic>E. coli</i> in the United States, which was not seen using the genotypic ESBL rates. The majority of false-positive phenotypic profiles had ceftazidime MICs of 2 µg/ml and a ≥3 doubling dilution decrease in MIC for only one of the two antimicrobial agents. False-positive ESBL results can adversely impact epidemiological surveillance and patient care (including inappropriate treatment, unnecessary patient isolation, and higher costs). Careful evaluation and comparison of phenotypic and genotypic test results can yield the ...
Source: Journal of Medical Microbiology - Category: Microbiology Authors: Tags: J Med Microbiol Source Type: research