This article is more than 5 years old. Information may no longer be current.
Imipenem elevated-MIC values increase from 2000 to 2014
BOSTON — A 14-year analysis of minimum inhibitory concentration, or MIC, data for imipenem showed that the proportion of Enterobacteriaceae isolates with low-MICs appears to be decreasing while isolates with elevated-MICs are increasing.
Researchers from the David Geffen School of Medicine at the University of California, Los Angeles Health System, reported that the increase in carbapenem-resistant Enterobacteriaceae in the United States remains a “significant public health concern.” They noted that in recent years, however, there also has been an “upward shift” in carbapenem MICs among susceptible isolates.
To further investigate this shift, the researchers performed antimicrobial susceptibility testing with imipenem (IPM) on 14,548 Enterobacteriaceae isolates collected over a 14-year duration. Several species, including Proteus, Morganella, and Providencia, were excluded from the analysis because of their natural elevated IPM MICs. The researchers defined “low-MIC” values as those less than or equal to 0.25 µg/mL, and “elevated-MIC” values as those ranging from 0.5 µg/mL to 1 µg/mL.
According to the results, there was a significant decrease in the proportion of low-MIC isolates (P = .004).
“We hypothesized this may be due to the expansion of isolates that harbor extended spectrum beta-lactamases that have weak activity against the carbapenems, including IPM,” the researchers wrote.
In 2001 and 2002, 77.4% of Enterobacteriaceae had low-MIC values for IPM, and 21.1% of isolates had elevated-MIC values. In 2013 and 2014, the percentage of low-MICs decreased to 68.8%, and elevated-MICs increased to 27.8%. The shift was mainly evident among isolates collected from outpatients (P = .004) and patients in the ICU (P = .006).
The researchers found that Klebsiella pneumoniae isolates were particularly affected by the IMP MIC shift, with 95.8% of isolates having low-MIC values in 2001 and 2002. That value dropped to 77.7% in 2013 and 2014 (P = .0002).
Further analyses revealed that isolates with elevated-MICs were associated with increased resistance to other cell wall-targeting agents, including ampicillin/sulbactam (28.2% vs. 18.7%), cefazolin (27.6% vs. 17%), and ceftazidime (14.5% vs. 10.4%).
While the study shows a shift in IPM MICs, Romney Humphries, PhD, assistant professor of pathology and laboratory medicine at UCLA, told Infectious Disease News, that similar trends also were observed for ertapenem and meropenem.
“This increase in MICs over time included not only an increase in the proportion of isolates with MICs at the high end of the susceptible category, but also in the number of isolates with intermediate MICs, which is concerning from a treatment perspective,” she said. While carbapenemase producers have been a major focus in recent years, subtle increases in MICs due to other resistance mechanisms is of concern, as it may limit the use of these antimicrobials in upcoming years."
The researchers concluded that a larger, multicenter trial is needed to verify the shift in IPM MICs, and that additional studies are warranted to better define the mechanism of this shift. – by Stephanie Viguers
Reference:
Lee MJ, et al. Imipenem MIC shift among carbapenem-susceptible Enterobacteriaceae: A single center metadata analysis between 2001 and 2014. Presented at: ASM Microbe; June 16-20, 2016; Boston.
Disclosure: The researchers report no relevant financial disclosures.