Ann Lab Med 2017; 37(4): 305-312  
Evaluation of Six Phenotypic Methods for the Detection of Carbapenemases in Gram-Negative Bacteria With Characterized Resistance Mechanisms
Kunling Sun, M.D., Xiuyu Xu, M.D., Jinrong Yan, M.D., and Liping Zhang, M.D.
Department of Laboratory Medicine, The First Affiliated Hospital of Chongqing Medical University, Chongqing, China
Correspondence to: Liping Zhang
Department of Clinical Laboratory, The First Affiliated Hospital of Chongqing Medical University, No.1 Youyi Road, Yuzhong District, Chongqing 400016, China
Tel: +86-023-89012756
Fax: +86-023-89012513
Received: May 31, 2016; Revised: December 16, 2016; Accepted: February 22, 2017; Published online: July 1, 2017.
© The Korean Society for Laboratory Medicine. All rights reserved.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Background: We compared the performance of the modified Hodge test (MHT), Triton Hodge test (THT), Carba NP test (CNPt), simplified Carba NP test (CNPt-direct), blue-Carba NP test (BCT), and carbapenem inactivation method (CIM) for rapid and accurate carbapenemase detection.
Methods: The methods were evaluated by using 256 gram-negative isolates, including 197 Enterobacteriaceae (79 Enterobacter spp., 74 Klebsiella spp., 33 Escherichia coli, 10 Citrobacter spp., and 1 Serratia marcescens), 51 Acinetobacter baumannii, and 8 Pseudomonas aeruginosa strains. The collection included 117 non-carbapenemase, 18 Klebsiella pneumoniae carbapenemases (KPC) producers, 46 New Delhi metallo-β-lactamases (NDM) producers, 11 imipenemases (IMP) producers, and 51 oxacillinases (OXA) producers, and 13 strains harboring two different carbapenemase genes.
Results: The specificity of the THT (91.5%) was significantly lower than other methods, each of which had 100% specificity (P<0.003). This can be attributed to the false detection of Ampler class C β-lactamases (AmpC) carriers. The CNPt-direct and CIM yielded the highest sensitivities (P<0.003), which were comparable (92.8% vs 93.5%, P>0.999). Because of improved detection of NDM carriers, THT showed significantly higher sensitivity than the MHT (84.9% vs 75.5%, P<0.001). However, poor performances in detecting OXA still influenced the sensitivities of the CNPt (66.2%) and BCT (82.0%), as well as the MHT and THT.
Conclusions: CNPt-direct and CIM demonstrated the best performance for the efficient detection of carbapenemase among the six evaluated methods. Except the MHT and THT, the detection of carbapenemase-producing Enterobacteriaceae by all the other methods was acceptable, when the OXA-type carbapenemase was not prevalent.
Keywords: Evaluation, Phenotypic methods, Carbapenemase, Gram-negative bacteria

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