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Determining carbapenem resistance
Resistance to antibiotic therapy via production of the enzyme carbapenemase by Klebsiella pneumoniae and other members of Enterobacteriaceae is becoming more common. This resistance is not always detected by conventional antimicrobial susceptibility testing, which may result in inappropriate antimicrobial therapy for the patient.
In Enterobacteriaceae, the gene blaKPC, which encodes KPC (Klebsiella pneumoniae carbapenemase) production, can be detected by real-time PCR. However, molecular methods have not been established for other carbapenemases.
The modified Hodge test, a phenotypic method, is recommended by the Clinical and Laboratory Standards Institute (CLSI) as the method to detect carbapenemases.
A positive result indicates the production of carbapenemase.
A negative result indicates the lack of production of carbapenemase.
This test is not routinely performed on Enterobacteriaceae determined to be fully resistant to carbapenems in the Mayo Microbiology Laboratory.
A collection of 14 isolates (11 Klebsiella pneumoniae and 1 each Enterobacter cloacae, Citrobacter freundii, and Escherichia coli) obtained from the CDC were studied for the detection of carbapenemase production. Eleven (including the Enterobacter cloacae, Citrobacter freundii, and Escherichia coli isolates) were positive and 3 were negative with Mayo's modified Hodge test and with the Mayo-developed KPC real-time PCR assay, as well as with modified Hodge testing and the KPC real-time PCR assay performed at CDC.
To further support this data, an additional 62 specimens were tested comparing Mayo's modified Hodge test to the Mayo-developed KPC real-time PCR assay. The modified Hodge test and the real-time PCR assay showed 100% concordance for both positive (n=10) and negative (n=52) paired results.
The 2009 CLSI Standards for Antimicrobial Susceptibility Testing. CLSI Audioconference. Janet Hindler. Original air date: January 21, 2009