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Reevaluating Susceptibility Breakpoints

After the Clinical and Laboratory Standards Institute (CLSI) revised their clinical breakpoints for susceptibility test results, a laboratory at a major medical center in a large midwest city evaluated enterobacterales as it relates to bloodstream infections (BSI) and piperacillin/tazobactam.


Enterobacterales is a gram-negative bacteria that can be a common cause for hospitalized patients who have BSI. Treatment using piperacillin/tazobactam has been shown to be challenging and not preferred.

Laboratory professionals have to take into account the revised changes organizations such as CLSI make and how they interpret results.

“One of the things about why it is important for laboratories to constantly update their breakpoints when they are revised by organizations like CSLI is that it has a direct impact on the results we provide to the clinician,” Nicholas Moore, PhD, D(ABMM), associate professor at Rush University and associate director in the Division of Clinical Microbiology, Departmentof Pathology, and director of the Biocontainment Laboratory at Rush University Medical Center, said in an interview with Contagion. “If we fail to update our breakpoints in a timely manner, then we are going to be putting out erroneous or inaccurate results that could lead to a patient being treated with an antibiotic that is really no longer effective for that type of infection.”

Moore and coinvestigators wanted to evaluate Enterobacterales blood isolates and the antimicrobial susceptibility testing looking at piperacillin/tazobactam. The investigators retrospectively evaluated 1597 Enterobacterales blood isolates over a five-year period.

Moore presented the data in a poster, “Impact of Revised Piperacillin/Tazobactam Clinical Breakpoints on Enterobacterales Isolates Identified in Blood Cultures,” at ID Week.

“Most isolates identified were Escherichia coli [n=806 (50%)] and Klebsiella pneumoniae [n=358 (22%)],” the investigators reported. “The majority of isolates (90.4%) were susceptible; 28 (1.8%) isolates were susceptible-dose dependent and 125 (7.8%) were resistant to piperacillin/tazobactam using the new CLSI breakpoints.”

“Even though the breakpoint for susceptibility for Enterobacterales for the drug piperacillin/tazobactam was lowered by the CSLI, overall when we looked at a large number of our isolates over a five year period the overall results or the number of isolates didn’t change,” Moore said.

Moore sat down to delve further into the study’s results, and provided some insights on the takeaways in the study, and the role of diagnostic stewardship in getting time to optimal therapy.