Phenotypic and Genotypic Evaluation of Carbapenemases in Carbapenem-Resistant Klebsiella pneumoniae Isolates and Investigation of Ceftazidime-Avibactam and Colistin Susceptibility

Carbapenem-resistant Klebsiella pneumoniae (CR-Kp) is a major healthcare-associated pathogen, and the coexistence of different carbapenemases may complicate phenotypic detection and antimicrobial treatment. This study evaluated carbapenemase profiles in 123 CR-Kp isolates from two centers in Türkiye and compared the modified carbapenem inactivation method (mCIM) and EDTA-modified carbapenem inactivation method (eCIM) with multiplex real-time PCR targeting KPC, NDM, VIM, IMP, and OXA-48. Ceftazidime-avibactam (CZA) and colistin susceptibility were also assessed. Carbapenemase genes were detected in 122/123 isolates (99.2%), predominantly NDM + OXA-48 (45.5%), followed by OXA-48 alone (26.8%), KPC alone (18.7%), and NDM alone (8.1%). mCIM showed 99.2% sensitivity, whereas eCIM showed 87.7% sensitivity and 100% specificity for NDM detection. All eight false-negative eCIM results occurred among NDM + OXA-48 co-producing isolates (8/55), whereas no false-negative results were observed among isolates carrying NDM alone (0/10); this difference was not statistically significant (p = 0.339). CZA resistance was observed in 49.6% of isolates, with a higher likelihood of resistance among NDM-positive isolates in the exploratory analysis (OR, 9.03; 95% CI, 3.97–20.54; p < 0.001). Colistin resistance was observed in 36.6% of isolates, with no statistically significant association with carbapenemase profiles in the analyses performed. These exploratory findings suggest that combined molecular and phenotypic testing may be useful for carbapenemase characterization and highlight the occurrence of false-negative eCIM results in some NDM + OXA-48 co-producing isolates. Given the absence of adjustment for multiple comparisons, the observed associations should be interpreted cautiously and considered hypothesis-generating.