Soon to be required: current susceptibility testing breakpoints

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Valerie Neff Newitt

October 2023—A CAP accreditation program requirement that microbiology laboratories use current antimicrobial susceptibility testing breakpoints, which was added to the checklist in 2021, will go into effect Jan. 1, 2024.

Three other microbiology laboratory requirements in the 2023 checklist edition, released in August, are new, expanded, or revised.

The antimicrobial susceptibility testing requirement, MIC.11385, calls for laboratories to use current breakpoints for the interpretation of antimicrobial minimum inhibitory concentration and disk diffusion test results. For laboratories subject to U.S. regulations, new breakpoints must be implemented within three years of the date of publication of an update by the Food and Drug Administration.

For laboratories not subject to U.S. regulations, new breakpoints must be implemented within three years of publication of an update by the standards development organization the laboratory uses, such as the Clinical and Laboratory Standards Institute or European Committee on Antimicrobial Susceptibility Testing.

“This is an important and big change,” says Carol A. Rauch, MD, PhD, adjunct associate professor of pathology, microbiology, and immunology at Vanderbilt University School of Medicine, a member of the CAP Checklists Committee, and a member until recently of the CAP Microbiology Committee.

Dr. Rauch

“That’s why laboratories were given advance notice,” she says. “This gave them time to read it, understand it, and think about what it means as well as what they have to do to comply.”

The reason for the new requirement: “Organisms are evolving faster than our regulatory framework has evolved,” she says.

“Imagine the FDA approved a device 15 years ago with an automated instrument, panels, and software, and your lab still uses that device to test and issue results. In the meantime, organisms have developed resistance, and there’s new data about outcomes and pharmacokinetics, findings that can lead to a change in breakpoints used to interpret results. When this happens, the old breakpoints are then considered obsolete.” If the lab fails to detect resistance and reports to the clinician that the organism is susceptible to the drug, the clinician may choose that drug for therapy and the patient may not do well because the reported test result was erroneous.

“Unfortunately, nothing has compelled device manufacturers to update breakpoints for their systems, and labs often rely on manufacturers for guidance,” Dr. Rauch says.

“If we’re using a device that was FDA approved or cleared a long time ago with breakpoints that were current at that time, and underneath us the ground has changed, patients are at risk.”

Dr. Rauch recently worked with the CDC’s Antibiotic Resistance Laboratory Network, where she observed that efforts to track resistance and contain outbreaks are hindered if the laboratory did not use current breakpoints.

“The resistance is not detected, and that resistant organism can spread to other patients within a facility and beyond as the patient moves to long-term care and then back to the ICU, et cetera. The labs doing the initial testing really have to do that right in order for our systems to work at containing resistance and making sure infections are treated appropriately.”

Step one for labs is to “know your breakpoints,” she says. “You can’t implement new ones until you’ve identified which ones need it.” A phase two requirement in the microbiology checklist, MIC.11380 Antimicrobial Susceptibility Test Interpretation Criteria, says there must be written criteria for determining and interpreting MIC or zone diameter sizes as susceptible, intermediate, resistant, nonsusceptible, or susceptible dose-dependent, and the criteria must be reviewed annually. This requirement was significantly revised in 2021 and, unlike MIC.11385, went into effect at that time.