Paradoxical carbapenemase activity detected by modified carbapenemase inactivation (mCIM) method in Citrobacter sedlakii

This case study describes a Citrobacter sedlakii isolate recovered during carbapenemase surveillance that tested strongly positive by the modified carbapenem inactivation method (mCIM), suggesting carbapenemase production, despite being phenotypically susceptible to carbapenems. Follow-up testing using eCIM, liquid chromatography mass spectrometry, and whole genome sequencing revealed that the large inoculum size used in the mCIM assay likely contributed to a false-positive result. These findings underscore that relying solely on mCIM and eCIM without appropriate susceptibility testing can lead to misinterpretation and erroneous detection of carbapenemase activity in non-resistant isolates.