Rapid Detection Methods for Carbapenemase Producing Gram-negative Bacilli with Reference to Phenotypic Carba M Test: A Cross-sectional Hospital based Study
Authors: Perumal P Ganesh , Kumar B Dhanush , Priyadharsini T , Appalaraju B , Someshwaran R 
Published: September 2024

Introduction: Carbapenem-resistant Gram-negative Bacilli (CR-GNB) cause many serious infections, resulting in increased treatment costs, prolonged hospitalisation, and a high mortality rate among infected patients. The survival of carbapenemaseproducing Gram-negative organisms is enhanced by mechanisms such as the presence of carbapenemases, reduced expression, and porin mutations. Accurate tests for rapid detection of bacterial antibiotic resistance are crucial for tracking, preventing, and containing the spread of resistant genes within hospitals and communities, as well as for guiding therapy. Aim: To evaluate the performance of the ‘Carba M test’ compared to phenotypic Carba NP tests, Carbapenem Inactivation Method (mCIM/eCIM) tests, and genotypic Carba R tests for the phenotypic detection and characterisation of various types of carbapenemase-producing GNB. Materials and Methods: A cross-sectional study was conducted in the Department of Microbiology at PSG Hospitals, Coimbatore, Tamil Nadu, India which has 1400 beds. The study was carried out over a period of 15 months (from June 2022 to August 2023). A total of 250 CR-GNB were included as the study population. A comparative evaluation of the Carba M test, Carba NP test, Carbapenem Inactivation Method (mCIM/eCIM), and Carba R test for the rapid detection of carbapenemase-producing GNB using standard methods was done. The data were analysed, and odds ratios and p-values were calculated for statistical significance at a 95% Confidence Interval (CI). Results: A total of 250 CR-GNB were isolated during the study period, of which 83 (33.2%) were from the Medical Intensive Care Unit (MICU), followed by the Medicine department with 24 (9.6%). Enterobacteriaceae accounted for the majority of the isolates, with 229 (91.6%), followed by Pseudomonas aeruginosa with 17 (6.8%) and Acinetobacter species with 4 (1.6%). Klebsiella pneumoniae was the most common isolate among Enterobacteriaceae, accounting for 216 (86.4%). Out of the 250 CR isolates tested by the Carba R test, 100 (40%) were positive for the Oxacillinase (OXA)-48 enzyme, 35 (14%) for the New Delhi Metallobetalactamase (NDM) enzyme, 4 (1.6%) for Verona Integron-encoded Metallobetalactamase (VIM), and all were negative for Imipenemase (IMP), and Oxacillinase (OXA), and Klebsiella pneumoniae Carbapenemase (KPC) genotypes. The sensitivity of mCIM/eCIM observed in the study was 67.12%, and specificity was 85.71% when compared to the molecular genotype (GeneXpert® Carba R test). The modified Carba NP test detected carbapenemases in 229 isolates, yielding an overall sensitivity of 95.07% and a specificity of 74.07%. The overall sensitivity and specificity of the Carba M test for detecting carbapenemase-producing GNB were 97.31% and 70.37%, respectively. The isolates producing Class A/D (OXA-48-like) enzymes were identified with sensitivities of 98% and specificities of 73.68%, while those producing Class B (NDM, VIM) enzymes showed sensitivities of 94.87% and specificities of 87.5%. Conclusion: The Carba M test is a cost-effective, feasible, and rapid phenotypic test for detecting and differentiating Class B from Class A/D carbapenemases. The Carba M test could serve as a supplemental test alongside the Carba R, Carba NP test, or mCIM/eCIM for diagnosing carbapenemase production in GNB.