Detection of carbapenemase-producing Pseudomonas aeruginosa: Evaluation of the carbapenem inactivation method (CIM)

Abstract

Introduction: The carbapenem inactivation method (CIM) is a cost-effective assay for detecting carbapenemases. However, its interpretation is unclear for Pseudomonas spp. We evaluate its accuracy when meropenem is changed to imipenem. Methods: We analyzed 266 P. aeruginosa isolates. The CIM method consists of: resuspend bacterial colonies (a full 10 μL loop) in 400 μL water, in which a 10 μg disk of meropenem/imipenem is immersed. After 2 h of incubation (35 °C), remove the disk, place it onto a Mueller-Hinton agar plate previously inoculated with Escherichia coli (ATCC 25922), and incubate at 35 ̊C between 18-24 h. Interpretation criteria (mm of inhibition zone): ≤19 mm, positive; ≥25 mm negative; 20–24 mm, undetermined. Results: Imipenem improves the sensitivity and specificity of CIM when compared to meropenem (99.4% and 98.9%, vs. 91.9% and 94.7%, respectively). Conclusions: The accuracy of CIM for carbapenemase detection in P. aeruginosa is increased with the use of imipenem.