Study finds CIM assays misclassify OXA-48-like CRE isolates

In a clinical study published on October 30, 2026, researchers examining 172 genotypically confirmed carbapenemase-encoding carbapenem-resistant Enterobacterales isolates from intensive care units in South India found that the modified carbapenem inactivation method and EDTA-modified CIM frequently misclassified OXA-48-like-producing isolates as metallo-beta-lactamase producers or carbapenemase-negative, lifescience.net reported.

Isolates and Resistance Profiles in Intensive Care Units

Carbapenem-resistant Enterobacterales represent an increasing public health concern globally. The study evaluated perirectal swabs collected from patients admitted to two intensive care units at a hospital in South India. Among the 172 confirmed isolates, Escherichia coli accounted for 53.5% of the samples, Klebsiella pneumoniae comprised 41.3%, and Enterobacter species made up 5.3%.

Genotypic testing revealed that 112 isolates, or 65.1%, encoded NDM alone. Furthermore, 23 isolates, representing 13.4%, encoded OXA-48-like enzymes alone. An additional 37 isolates, or 21.5%, co-encoded both NDM and OXA-48-like carbapenemases.

Phenotypic Test Performance Across Gene Variants

Performance metrics varied substantially depending on the specific genetic makeup of the isolates. Modified carbapenem inactivation method positivity reached 98.2% for isolates encoding NDM alone. For OXA-48-like encoders, positivity dropped to 47.8%. Isolates co-encoding NDM and OXA-48-like showed a 94.6% positivity rate.

Among the isolates that tested positive on the modified carbapenem inactivation method, EDTA-modified CIM positivity rates followed a different pattern. Rates reached 99.1% for NDM encoders, 91% for OXA-48-like encoders, and 97.1% for isolates carrying both genes.

Misclassification Risks for Dual-Encoding Strains

When evaluated against molecular carbapenemase gene detection, the phenotypic assays demonstrated high sensitivity for classifying NDM-encoding isolates as metallo-beta-lactamase producers. However, the tests frequently misclassified isolates encoding OXA-48-like genes. These strains were incorrectly categorized either as having a metallo-beta-lactamase or as lacking a carbapenemase entirely.

For the collection of NDM and OXA-48-like co-encoding isolates, which represents the largest group of such strains analyzed to date, the assays typically reported the samples as metallo-beta-lactamase producers. These findings indicate that assay performance can fluctuate due to variable activity, distribution, and combinations of carbapenemases across different geographic regions.