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

TLDR

Reliable detection of carbapenemase‑producing Enterobacteriaceae is critical for containment, yet existing phenotypic methods have limitations, prompting the development of the carbapenem inactivation method (CIM). This study evaluates a modified CIM (mCIM) that replaces water with tryptic soy broth during inactivation and extends incubation time. We performed a single‑laboratory validation to assess accuracy, then a nine‑laboratory study testing isolates with known carbapenemase genes to determine reproducibility and optimal zone‑size cutoff. The mCIM achieved 99 % sensitivity and 100 % specificity in validation, with nine‑lab sensitivities ranging 93–100 % (mean 97 %) and specificities 97–100 % (mean 99 %), delivering results in under 24 h with excellent reproducibility.

Abstract

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.