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Prevalence and Significance of a Negative Extended-Spectrum β-Lactamase (ESBL) Confirmation Test Result after a Positive ESBL Screening Test Result for Isolates of Escherichia coli and Klebsiella pneumoniae: Results from the SENTRY Asia-Pacific Surveillance Program

ABSTRACT A negative extended-spectrum β-lactamase (ESBL) confirmation test result obtained after a positive ESBL screening test result using Clinical and Laboratory Standards Institute methods has been a common occurrence among isolates of Escherichia coli and Klebsiella pneumoniae in the SENTRY Antimicrobial Surveillance Program in the Asia-Pacific region. Among isolates collected between 1998 and 2004 this screen-positive, nonconfirmed profile (failed to show clavulanate synergy) was observed in 8.9% of 4,515 E. coli isolates and 20.3% of 2,303 K. pneumoniae isolates. We then selected 52 E. coli isolates and 68 K. pneumoniae isolates with a negative ESBL confirmation test, as well as comparable number of isolates with confirmed ESBL-positive tests, and examined them for the presence of TEM, SHV, plasmid-borne ampC, and CTX-M genes. We found that 62% of nonconfirming E. coli isolates and 75% of nonconfirming K. pneumoniae harbored a plasmid-borne AmpC enzyme of the CIT or DHA type. The majority of nonconfirming E. coli and K. pneumoniae from the Asia-Pacific region harbor important β-lactamases, and a positive screening test alone should be sufficient grounds to report resistance to extended-spectrum cephalosporins in this region.