Use of MALDI-TOF VITEK MS for rapid and efficient identification of KPC-type carbapenemases in Enterobacterales carrying the Tn4401a transposon.

Purpose: To determine diagnostic validity of MALDI-TOF MS (VITEK MS) system for detecting Klebsiella pneumoniae carbapenemases (KPC)-type carbapenemases by identifying the 11,109 Da peak in the mass spectrum generated for species identification as compared to RAPIDEC^® CARBA NP, and the modified carbapenemase inactivation method (mCIM) and the EDTA-modified carbapenem inactivation method (eCIM) in a collection of isolates previously characterized as KPC positive or negative.

Methods: 210 Enterobacterales clinical strains previously characterized having bla_KPC gene, the pKpQIL plasmid and the Tn4401a transposon were evaluated, including 34 positive controls carbapenemase-producing Klebsiella pneumoniae associated with Tn4401a, 30 Enterobacterales bla_KPC positive of unknown plasmid background, and 146 negative controls. Accuracy and agreement were established for Vitek MS, RAPIDEC^® CARBA NP, and mCIM/eCIM) tests; ROC curves were compared among these tests.

Results: The 11,109 Da peak was detected in 100% of KPC Tn4401a positive isolates using Vitek MS, sensitivity of 100% (95% CI 98.53-100), specificity of 95.5% (95% CI 91.7-99.4), positive predictive value (PPV) of 85.0 (95% CI 72.7-97.3), negative predictive value (NPV) of 100% (95% CI 99.6-100) and positive Likelihood Ratio (PLR) of 22.3 (10.2-48.8). Agreement between the three tests was 93.3% Kappa index of 0.90 (95% CI 0.83-0.97, p ≤ 0.05). ROC curves showed areas under the curve (AUCs) of 0.95, 0.96 and 0.96 for the VITEK MS, RAPIDEC CARBA NP and the mCIM/eCIM tests, respectively.

Conclusion: Detection of the 11,109 Da peak by Vitek MS confirms the presence of KPC-type carbapenemase, allowing rapid and simultaneous detection with species identification; a negative result does not rule out the presence of the enzyme and may require additional tests.