Modified Carbapenem Inactivation Method for Phenotypic Detection of Carbapenemase Production among Enterobacteriaceae

Abstract

The ability of clinical microbiology laboratories to reliably detect carbapenemase-producing carbapenem-resistant Enterobacteriaceae (CP-CRE) is an important element of the effort to prevent and contain the spread of these pathogens and an integral part of antimicrobial stewardship. All existing methods have limitations. A new, straightforward, inexpensive, and specific phenotypic method for the detection of carbapenemase production, the carbapenem inactivation method (CIM), was recently described. Here we describe a two-stage evaluation of a modified carbapenem inactivation method (mCIM), in which tryptic soy broth was substituted for water during the inactivation step and the length of this incubation was extended. A validation study was performed in a single clinical laboratory to determine the accuracy of the mCIM, followed by a nine-laboratory study to verify the reproducibility of these results and define the zone size cutoff that best discriminated between CP-CRE and members of the family Enterobacteriaceae that do not produce carbapenemases. Bacterial isolates previously characterized through whole-genome sequencing or targeted PCR as to the presence or absence of carbapenemase genes were tested for carbapenemase production using the mCIM; isolates with Ambler class A, B, and D carbapenemases, non-CP-CRE isolates, and carbapenem-susceptible isolates were included. The sensitivity of the mCIM observed in the validation study was 99% (95% confidence interval [95% CI], 93% to 100%), and the specificity was 100% (95% CI, 82% to 100%). In the second stage of the study, the range of sensitivities observed across nine laboratories was 93% to 100%, with a mean of 97%; the range of specificities was 97% to 100%, with a mean of 99%. The mCIM was easy to perform and interpret for Enterobacteriaceae, with results in less than 24 h and excellent reproducibility across laboratories.

Is This the Carbapenemase Test We've Been Waiting for? A Multicenter Evaluation of the Modified Carbapenem Inactivation Method

“…Pierce and colleagues (19) found the mCIM assay to have excellent sensitivity and specificity: 23 of 27 known carbapenemase-producing Enterobacteriaceae isolates gave positive results by the mCIM assay at all nine laboratories with the three additional isolates positive in 8 of the 9 laboratories. Similar results were observed for the 34 isolates known to be negative for carbapenemases.…”

“…In this issue of the Journal of Clinical Microbiology, Pierce and colleagues describe a multicenter evaluation of a modified carbapenem inactivation method assay (mCIM) (19). The original carbapenem inactivation method (CIM), first published by van der Zwaluw and colleagues, is based on a simple principle using reagents readily available to clinical microbiology laboratories (20), namely, that the contents of a 10-g meropenem disk are degraded through enzymatic activity when the disk is incubated in a bacterial suspension of carbapenem-producing organism.…”

“…To improve sensitivity for the detection of carbapenemases that are expressed at low levels or that naturally have weaker hydrolytic activities, the mCIM assay uses less organism (1-l versus 10-l loopful), a longer incubation period of the disk-bacterial suspension (4 h versus 2 h), and a different suspension matrix (tryptic soy broth versus water) than the original CIM assay. Importantly, Pierce et al (19) describe and validate diffusion cutoffs that include "indeterminate" interpretive criteria. This is the first multicenter evaluation of the mCIM method and the only publication to evaluate the reproducibility of the assay.…”

Clinical and genomic epidemiology of carbapenem-non-susceptibleCitrobacterspp. at a tertiary healthcare center over two decades

“…Only about half of the isolates (55%, 11/20) were non-susceptible to meropenem, with three isolates having intermediate resistance and eight being resistant (Table 2). We tested all isolates for the presence of a carbapenemase using a modified carbapenem inactivation (mCIM) test (23), for which 13 isolates (65%) tested positive. Carbapenemase genes were present in the genomes of all 13 isolates (Table 2).…”

“…Susceptibility to additional agents was determined by the broth microdilution method (20). Presence of carbapenemase enzyme activity was assessed by modified carbapenem inactivation (mCIM) test (23).…”