News for EUCAST 2022

EUCAST always publishes yearly updates of their bacterial breakpoint tables document¹ on January 1^st. Areas of the document that have been updated are also highlighted to support rapid uptake of these newest edits. The following text summarizes some of the most notable changes made in 2022. 

EUCAST adds brackets to a breakpoint to indicate that for a certain combination of bacterial species and antibiotic there are not sufficient data to allow a clinical breakpoint to be set², and is therefore based on the ECOFFs of the organisms, that differentiate between strains without and with a resistance mechanism. If the antibiotic is often used clinically or is among the ultimate options for treatment, in other words, guidance for treatment is desirable, then a bracketed breakpoint is used to indicate that the agent should be used only on specifically documented indications, or in combination with other active therapy. Other active therapy can be a second antibiotic as well as, for example, surgical intervention. The result of a susceptibility test should not be reported as S or I, since both would imply that there are clinical data suggesting a high probability of clinical cure. Instead a comment is recommended that indicates the need for combination therapy. A “Guidance Document” on the EUCAST website outlines the background and recommendations for handling², but essentially, only a resistant result may be reported as such.

Since the joint committee for colistin testing of CLSI and EUCAST has reached the conclusion that for this drug the exposure of the microorganisms often is not sufficient to allow characterisation as “S, susceptible” or “I, susceptible, increased exposure”, EUCAST has decided to present these breakpoints in brackets only. The solution taken by CLSI to use an “I, intermediate” is not possible for EUCAST since the definitions of the “I” differ between the systems³.

Re-analysis of available data indicate that zone diameters for meropenem disk diffusion differ between P. aeruginosa and other Pseudomonas spp., so the breakpoints for disk diffusion now differ.

For detection of oxacillin resistance in staphylococci the breakpoint for the cefoxitin screen test in S. epidermidis and S. lugdunensis is now different from that for S. aureus and other coagulase-negative staphylococci. For S. pseudintermedius, S. schleiferi and S. coagulans the oxacillin disk test should be used for screening.

In the 2022 tables EUCAST has published a disk diffusion method4 for susceptibility testing of some anaerobic bacteria. During development of the procedure it became obvious that some of the previous breakpoints for anaerobic bacteria had to be amended. Breakpoints are presented for Bacteroides spp., Prevotella spp., Fusobacterium necrophorum, Clostridium perfringens, Cutibacterium acnes, and Clostridioides difficile and the antibiotics benzylpenicillin, piperacillin-tazobactam, meropenem, clindamycin, metronidazole, and vancomycin.

Both MIC determination and disk test approaches use FAA (fastidious anaerobe agar) and incubation in an anaerobic atmosphere that may be generated with several of the generally available methods. For disc diffusion zones must be read after 16-20 h and it is not possible to prolong incubation. Reading of zones needs some training and it is very important to dry the plates before inoculation. Documents that describe the procedure and a reading guide are available on the website⁴.

1. https://www.eucast.org/clinical_breakpoints/ 
2. https://www.eucast.org/eucastguidancedocuments/ 
3. https://www.eucast.org/newsiandr/ 
4. https://www.eucast.org/ast_of_bacteria/disk_diffusion_methodology/ 

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