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Confirmation of carbapenemase production from pure isolates of Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter species
Gram-negative bacilli (GNB) with acquired carbapenemases have disseminated worldwide, rendering them a global threat. The therapeutic armamentarium for infections caused by carbapenem-resistant Enterobacteriaceae (CRE) is limited, and CRE infections have been associated with significant mortality. Enterobacteriaceae harboring Klebsiella pneumoniae carbapenemase are endemic in some regions of the United States, and although still sporadic, GNB harboring New Delhi metallo-beta-lactamase have been reported from several states. Timely detection of these carbapenemases (along with emerging carbapenemases such as OXA-48 and VIM) is important. Detection is challenging since isolates may have only borderline reductions in susceptibility to carbapenems, and carbapenem resistance may be mediated by mechanisms other than carbapenemases (eg, AmpC or extended-spectrum beta-lactamase with decreased membrane permeability). While molecular methods are confirmatory, testing may not be immediately available and may be limited by the number of targets assayed. The modified Hodge test suffers from lack of specificity, a long turnaround time, and poor sensitivity for metallo-beta-lactamase detection. The Carba NP test is preferred over the modified Hodge test due to improved specificity and faster turnaround time.
The Carba NP test is more specific than and as sensitive as the carbapenemase-modified Hodge test. If an isolate is suspected to possess KPC or NDM carbapenemase (eg, due to local epidemiology), KPC and NDM PCR (KPNRP / KPC (blaKPC) and NDM (blaNDM) in Gram-Negative Bacilli, Molecular Detection, PCR) may be preferred over the Carba NP test.
A positive result indicates production of a carbapenemase by the isolate submitted for testing. A negative result indicates lack of production of a carbapenemase by the isolate submitted for testing.
Results of the Carba NP test should be interpreted along with antimicrobial susceptibility testing results. Phenotypic resistance to carbapenems may be due to traits other than carbapenemase production (eg, AmpC or extended-spectrum beta-lactamase production with decreased membrane permeability). Additionally, a positive test is only indicative of carbapenemase production in general; the assay does not determine the type of carbapenemase present (eg, NDM-1, KPC, OXA-48-like). If an isolate is suspected to possess KPC or NDM carbapenemase (eg, due to local epidemiology), KPC and NDM PCR (KPNRP / KPC (blaKPC) and NDM (blaNDM) in Gram-Negative Bacilli, Molecular Detection, PCR) may be preferred.
False-negative results may occur due to plasmid loss in isolates submitted for testing, the presence of a nonexpressed carbapenemase gene, or low-level carbapenemase expression.
We evaluated 271 Gram stain-negative bacilli (of which 131 were carbapenemase producers and of which 201 were Enterobacteriaceae) using the Carba NP test and the modified Hodge test. Sensitivity for detection of carbapenemase production was comparable (Carba NP, 100 versus modified Hodge test, 98%, p=0.08), but the Carba NP test was more specific (100 versus 80%, p<0.0001) and faster.(1)
1. Vasoo S, Cunningham SA, Kohner P, et al: Comparison of a novel, rapid chromogenic biochemical assay, the Carba NP test, with the modified Hodge test for detection of carbapenemase-producing Gram-negative bacilli. J Clin Microbiol 2013;51(9):3097-3101
2. Nordmann P, Poirel L, Dortet L: Rapid detection of carbapenemase-producing Enterobacteriaceae. Emerg Infect Dis 2012;18:1503-1507