Maria E. Navas, MD, and Michael R. Jacobs, MD, PhD, in the January issue of CLN, discuss how carbapenem-resistant Enterobacteriaceae (CRE) is transmitted in clinical settings, and what laboratories and physicians can do to identify infected patients and help prevent the spread of CRE.
“Enterobacteriaceae are part of the normal human gastrointestinal flora but are also commonly isolated from patients with urinary tract infections, as well as from hospitalized patients with blood stream infections and nosocomial pneumonia,” wrote Navas and Jacobs.
According to some reports, the mortality rate for CRE-related infections is as high as 50%. CRE are resistant to a long list of treatments, including carbapenems, penicillins, cephalosporins, aztreonam, ertapenem, imipenem, and meropenem. “CRE tend to show high levels of resistance to other classes of antibiotic agents due to the frequent occurrence of other resistance genes on the same mobile genetic elements,” the authors explained.
This is why it’s so important for labs to establish clear definitions and cutoff parameters to detect CRE. If test results are uncertain, additional testing using methods like the modified Hodge test (MHT), carba-NP test, and PCR tests for carbapenemases may be necessary, the authors emphasized.
Navas and Jacobs also recommended that microbiology labs offer susceptibility testing for some drugs that have no breakpoints recommended by the Clinical and Laboratory Standards Institute (CLSI) or the Food and Drug Administration (FDA). These may include drugs such as colistin and tigecycline, “as well as newer drugs with susceptibility dilution gradient strips or disks that are not yet FDA-cleared, such as ceftazime-avibactam. Each laboratory should establish a policy on how to test and report these agents,” the authors wrote.
Screening patients who might be carrying CRE strains in their gastrointestinal tract received specific attention in this article. “CRE screening can be done either universally on all new admissions to the hospital using rectal swabs or whenever positive cases are identified,” Navas and Jacobs indicated.
But as they highlight in the review, the various screening methods for CRE have their limitations, and some aren’t available yet for widespread use in the United States.
Although rectal swabs are the most frequently used screening method, standardization of the sample processing and result analysis for this method hasn’t been well established. “The Centers for Disease Control and Prevention (CDC) recommends culturing these specimens using traditional culture media. However, this process is lengthy and time consuming as it requires a selection/enrichment phase before plating and work-up,” they noted.
Technologies such as commercial chromogenic agars and a variety of rapid screening tests have yet to obtain FDA clearance for in vitro diagnostic use in the U.S. They are only available for research purposes. To date, not enough data has been published that evaluate their use and performance and, as the authors point out, not all of them are equipped to screen for all types of CRE.
“In exploring the best way to process and report these samples, laboratories need to collaborate with all parties involved, including infectious disease, infection control, pharmacy, and clinical and nursing staff,” Navas and Jacobs recommended.
Pick up the January issue of CLN and read more about the clinical implications of CRE, the environments where it might spread, and the protocols for labs to detect CRE.