Carbapenem Inactivation Method: Accurate Detection and Easy Interpretation of Carbapenemase Production in Enterobacteriaceae and Pseudomonas spp

Background: We evaluated the carbapenem inactivation method (CIM) compared with the modified Hodge test (MHT) for the detection of carbapenemase-producing Gram-negative bacilli. Methods: A total of 61 isolates of carbapenemaseproducing Enterobacteriaceae (CPE: 14 KPC, 7 GES5, 8 NDM-1, 9 VIM-2, 9 IMP-1, and 14 OXA-48-like), 34 isolates of metallo-β-lactamase (MBL)-producing Pseudomonas spp. (14 VIM-2 and 20 IMP-6), and 70 carbapenem-nonsusceptible carbapenemase-negative isolates were included. The CIM and MHT were performed for all of the isolates. To perform the CIM, a meropenem disk was incubated with a suspension of the isolate to be tested and then on Mueller-Hinton agar with the Escherichia coli ATCC 29522 strains. The absence of an inhibition zone indicates presence of a carbapenemase. The presence of a clearing zone indicates lack of a carbapenemase. Results: The total sensitivity and specificity of CIM (96% sensitivity and 100% specificity) in carbapenemnonsusceptible Enterobacteriaceae and Pseudomonas spp. were better than those of the MHT (77% sensitivity and 94% specificity). The interpretation of CIM results was easy, with no or <20 mm inhibition zones indicating positivity and >20 mm inhibition zones indicating negative carbapenemase activity. Conclusion: The CIM had excellent sensitivity and specificity for detection of CPE and MBL-producing Pseudomonas spp., and a positive result was easily determined, unlike the MHT. (Ann Clin Microbiol 2016;19:83-87)