Evaluation of the SpeeDx Carba (beta) multiplex real-time PCR assay for detection of NDM, KPC, OXA-48-like, IMP-4-like and VIM carbapenemase genes.

Background: Carbapenemase-producing organisms (CPOs) have emerged as antibiotic-resistant bacteria of global concern. Here we assessed the performance of the Carba (beta) assay, a multiplex real-time PCR assay developed by SpeeDx for the detection of key carbapenemase-encoding genes: KPC, NDM, OXA-48-like, IMP-4-like, and VIM.

Methods: DNA extracts of 180 isolates were tested with the Carba (beta) assay, using previously validated in-house TaqMan probe assays for the relevant carbapenemase genes as the reference standard. The Carba (beta) assay was then directly used to screen 460 DNA extracts of faecal specimens, with positive results subjected to the aforementioned in-house assays plus Sanger sequencing.

Results: The Carba (beta) assay correctly identified the presence of the respective carbapenemase genes in 154 of 156 isolates and provided negative results for all 24 non-CPO isolates. Two isolates provided positive results for OXA-48-like carbapenemase by the Carba (beta) assay only. The Carba (beta) assay had sensitivities of 100% for all targets, and specificities of 100% for KPC, NDM, IMP-4-like, and VIM targets, and 98.5% for OXA-48-like targets. When applied directly to faecal specimens, eight samples were positive by the Carba (beta) assay, two of which were confirmed by in-house TaqMan probe PCR or DNA sequencing.

Conclusions: The Carba (beta) assay is highly sensitive and specific for detecting key carbapenemase genes in isolates. Further testing is required to assess this assay's suitability for direct screening of clinical specimens.