Carbapenem Resistance in Enterobacteriaceae: Trends from Active, Population-Based Surveillance Using Automated Testing Instrument Queries in Hennepin and Ramsey Counties, Minnesota, 2013 - 2015

Background: Carbapenem-resistant Enterobacteriaceae (CRE) are an emerging public health threat in the United States. We describe trends in CRE using cases reported through active surveillance and the number of Enterobacteriaceae identified by automated testing instrument (ATI) queries from clinical laboratories in two Minnesota metropolitan counties from January 2013 through June 2015.

Methods: CRE are reportable to the Minnesota Department of Health in Hennepin and Ramsey Counties, as part of the Emerging Infections Program. The CRE definition we used was Citrobacter spp., Enterobacter spp., E. coli, and Klebsiella spp. that are non-susceptible to imipenem, meropenem, or doripenem and resistant to all third-generation cephalosporins tested using the Clinical Laboratory Standards Institute interpretive criteria. Clinical laboratories query their ATI by minimum inhibitory concentration values to identify cases. In addition, laboratories report ATI denominator queries indicating the number of Enterobacteriaceae by genus and species identified from clinical specimens. The percentage of Enterobacteriaceae of the same four species meeting the CRE definition was calculated from January 2013 through June 2015. Trends over time were analyzed by Chi-square for linear trend with corresponding p-values (Epi Info).

Results: There were 272 cases of CRE reported during the study period. ATI denominator queries identified 223,486 Enterobacteriaceae of the included species, for an overall CRE rate of 0.12%. CRE rates varied by organism; the rate of CRE was 0.43% in Citrobacter spp., 1.47% in Enterobacter spp., 0.02% in E. coli, and 0.16% in Klebsiella spp. There were no statistically significant trends identified over the 2.5 year period (p > 0.05 for each of the 4 species).

Conclusion: We found stable rates of close to 1.5% of carbapenem resistance among Enterobacter spp., in Minnesota, although CRE rates were much lower in other Enterobacteriaceae species. Using ATI queries to identify cases and denominators as part of an active, population-based surveillance system can readily enable monitoring of CRE trends, which can inform local prevention and control measures.