Mehrotra H, Favazza L, Kezlarian B, Fowler R, Hanson N, and Tibbetts R. Resolution of a modified CDC definition for carbapenem resistant enterobacteriaceae (CRE) using a rapid multiplex, cartridge-based molecular assay for the confirmation of carbapenemase genes. Modern Pathology 2020; 33(3):1880-1881.
Background: CRE have emerged as a global threat due to their potential to cause invasive infections, often with high mortality rates and are primarily healthcare, associated, with a potential for spread to community settings. The CDC updated its definition of CRE as resistant to a carbapenem: MIC of ≥4 ug/ml for doripenem, meropenem, or imipenem, ≥2 ug/ml for ertapenem, OR documented to produce a carbapenemase. CRE are resistant to carbapenems primarily through the expression of carbapenemase genes (CP-CRE) carried on plasmids, which are easily transferrable hence the CDC recommendation to isolate patients with CP-CRE. However, production of AmpC/ESBL β-lactamases with a decrease of outer membrane proteins (OMP) (non-CP-CRE) can also lead to a higher ertapenem MIC, of which OMPs are not transferable but meet the CDC definition for CRE. The purpose of this study was to develop an algorithm to differentiate CP-CRE from non-CP-CRE using carbapenem MIC and confirmation by a rapid molecular assay.
Design: Genetic and phenotypic testing was performed on 61 isolates of Enterobacteriaceae with MICs to ertapenem ranging from64 ug/ml using PCR to detect blaKPC, blaAmpC and blaESBL, and SDS-PAGE to determine OMP production. These data were used to create an algorithm to define CRE in our lab using MIC, the presence of resistance genes and/or OMP production. 40 isolates, including positive and negative controls, with known antibiotic susceptibilities were defined by this algorithm and confirmed by commercially available FDA approved rapid multiplex PCR for carbapenemase genes.
Results: Using this algorithm, we observed 5 discordant PCR results giving us 85% concordance. However, we believe that 3 of these were due to loss of plasmids by repeated freeze-thaw cycles and intend to reanalyze MICs to confirm this. On eliminating these 3 isolates, we have 93% concordance.(Table presented)
Conclusions: A combination of the CDC definition for CRE and lowered breakpoints to ertapenem led to overcalling non-CP-CRE as CPCRE may have resulted in inappropriate patient isolation, which is known to have negative patient outcomes and increase costs. Implementation of a multiplex PCR to rule out carbapenemases in isolates with elevated ertapenem but susceptible meropenem MICs resulted in a more sensitive and specific identification of CP-CRE.