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Diagnostic Performance of Combination Disk and CIM Tests for Carbapenemase Detection in Enterobacterales Compared
Aslı Çabuk1, Mustafa Zahir Bakıcı2, Cem Çelik1
1Department of Medical Microbiology, Sivas Cumhuriyet University School of Medicine, Sivas, Türkiye.
Objectives:
This study aimed to evaluate the diagnostic performance of the combination disk method and carbapenem inactivation method (CIM) for detecting carbapenemase production in Enterobacterales isolates with reduced susceptibility or resistance to carbapenems, using real-time multiplex polymerase chain reaction (PCR) as the reference method.
Materials And Methods:
Enterobacterales isolates obtained from clinical samples between 2015 and 2016, which were at least minimally susceptible or resistant to at least one carbapenem, were included in the study. Phenotypic evaluation was performed using the KPC&MBL&OXA-48 combination disk kit (Liofilchem, Italy), which detects Klebsiella pneumoniae carbapenemase (KPC), metallo-β-lactamase (MBL), and OXA-48-type carbapenemases. In addition, the CIM method was applied. For genotypic confirmation, the KPC, OXA-48, and New Delhi metallo-β-lactamase (NDM) genes were investigated using the BD MAXTM CRE Assay (Becton, Dickinson and Company, Sparks, MD, USA). Diagnostic accuracy parameters and the agreement coefficient (κ) were calculated for the index tests.
Results:
A total of 111 Enterobacterales isolates were analyzed. The majority of isolates were K. pneumoniae (n=80; 72.1%) and Escherichia coli (n=20; 18.0%). Carbapenemase genes were detected in 90 isolates by real-time multiplex PCR. The OXA-48 gene predominated (68.5%), while OXA-48+NDM (9.9%) and NDM (2.7%) were also detected; KPC was not detected. The sensitivity and specificity of the temocillin resistance-based phenotypic approach for predicting the presence of OXA-48 or OXA-48+NDM were 100.0% and 95.8%, respectively (κ=0.97). The MBL phenotype of isolates carrying NDM showed complete agreement with multiplex PCR for carbapenemase detection using the CIM test (sensitivity and specificity 100%, κ=1.00).
Conclusion:
The carbapenem inactivation method demonstrated high diagnostic accuracy for detecting carbapenemase production. The combination disk method, including temocillin resistance and ethylenediaminetetraacetic acid (EDTA)-based evaluation, showed strong performance in predicting OXA-48- and NDM-associated carbapenemases. However, in endemic regions, phenotypic results should ideally be confirmed by molecular methods.
