Evaluation of Carbapenemase Screening and Confirmation Tests with Enterobacteriaceae and Development of a Practical Diagnostic Algorithm

Abstract

Reliable identification of carbapenemase-producing members of the family Enterobacteriaceae is necessary to limit their spread. This study aimed to develop a diagnostic flow chart using phenotypic screening and confirmation tests that is suitable for implementation in different types of clinical laboratories. A total of 334 clinical Enterobacteriaceae isolates genetically characterized with respect to carbapenemase, extended-spectrum ␤-lactamase (ESBL), and AmpC genes were analyzed. A total of 142/334 isolates (42.2%) were suspected of carbapenemase production, i.e., intermediate or resistant to ertapenem (ETP) and/or meropenem (MEM) and/or imipenem (IPM) according to EUCAST clinical breakpoints (CBPs). A group of 193/334 isolates (57.8%) showing susceptibility to ETP, MEM, and IPM was considered the negative-control group in this study. CLSI and EUCAST carbapenem CBPs and the new EUCAST MEM screening cutoff were evaluated as screening parameters. ETP, MEM, and IPM with or without aminophenylboronic acid (APBA) or EDTA combined-disk tests (CDTs) and the Carba NP-II test were evaluated as confirmation assays. EUCAST temocillin cutoffs were evaluated for OXA-48 detection. The EUCAST MEM screening cutoff (<25 mm) showed a sensitivity of 100%. The ETP APBA CDT on Mueller-Hinton agar containing cloxacillin (MH-CLX) displayed 100% sensitivity and specificity for class A carbapenemase confirmation. ETP and MEM EDTA CDTs showed 100% sensitivity and specificity for class B carbapenemases. Temocillin zone diameters/MIC testing on MH-CLX was highly specific for OXA-48 producers. The overall sensitivity, specificity, positive predictive value, and negative predictive value of the Carba NP-II test were 78.9, 100, 100, and 98.7%, respectively. Combining the EUCAST MEM carbapenemase screening cutoff (<25 mm), ETP (or MEM), APBA, and EDTA CDTs, and temocillin disk diffusion on MH-CLX promises excellent performance for carbapenemase detection. In recent years, the emergence of diverse carbapenemases in members of the family Enterobacteriaceae has become a major challenge for health care systems (1). Carbapenemase-producing bacterial isolates pose a severe clinical problem, as nonsusceptibility to beta-lactams is frequently accompanied by coresistance to additional drug classes, e.g., aminoglycosides or quinolones (2, 3). As a consequence, treatment options for carbapenemase producers are alarmingly limited and often drugs displaying significant side effects need to be administered as a last resort (4).␤-Lactamases are classified according to their functional properties and molecular structure by Ambler and Bush (5, 6). Some of these enzymes also display hydrolytic activity toward carbapenems, e.g., Klebsiella pneumoniae carbapenemase (KPC, Ambler/ Bush class A)-, the New Delhi metallo-␤-lactamase (NDM-1)-, VIM-, and GIM-type enzymes (all Ambler/Bush class B) or OXA-48 (Ambler/Bush class D). A key characteristic used to discriminate enzymes belonging to different Ambler/Bush classes is responsiveness to specific inhibitors: class...

Evaluation of the Rapidec Carba NP Test for Detection of Carbapenemases in Enterobacteriaceae

“…The difference in sensitivity may be attributable to the different study populations, i.e., 20% (51/252 isolates) carbapenemase producers in the present study population versus 56% (98/173 isolates) in the study by Poirel and Nordmann (19). The Rapidec Carba NP test seems to provide greater sensitivity than the original "homemade" Carba NP I test, which was reported to display sensitivities of 72.5% to 100%, depending on the modification to the original protocol (11,28,29). The results of this study confirm previously reported findings pointing to ambiguities in the reading of the Carba NP II test for bla OXA-48 -positive isolates frequently producing false-negative results, as four of five false-negative results occurred with OXA-48-like enzymes (11).…”

“…Critical diameters or MICs for meropenem were recommended by EUCAST for carbapenemase screening and have been demonstrated to be highly sensitive (10,11). To confirm the production of carbapenemases, the CLSI recommends the modified Hodge test and the Carba NP I test (12).…”

“…To confirm the production of carbapenemases, the CLSI recommends the modified Hodge test and the Carba NP I test (12). Alternatively, several commercially available and laboratory-developed combined disk tests that use boronic acid and EDTA/dipicolinic acid as specific carbapenemase inhibitors are available (11,(13)(14)(15)(16). In its 2013 guidelines for the detection of carbapenemases, EUCAST recommended combined disk tests for carbapenemase confirmation (10).…”

Performance of Rapidec Carba Np Test in Detection of Carbapenemase Producing Gram-Negative Bacteria.

“…Maurer et al (20) also reported sensitivity of 78.9% for CPE detection by the Carba NP test. It was suggested that the sensitivity of the Carba NP test may depend on the prevalence of carbapenemases with low hydrolysis activity to imipenem in each area (particularly OXA carbapenemases or some IMP enzymes), low carbapenemase gene expression in some isolates, mucoid colonies (difficulties in protein extraction), species-specific traits (due to the unknown impact of different genetic backgrounds), types of media agar or imipenem powder.…”

“…Commercially available tests have several advantages over laboratory-developed assays, i.e., production is highly standardized and the quality is controlled by the manufacturer, reagents are preprepared and ready to use, and the shelf-life of the test kits is guaranteed (8) .…”