What's New in CLSI M100 for 2026? Key AST Updates Every Micro Lab Should Know
June 16, 2026
Written by
Laurie Bjerklie
Education Lead
If you work in clinical microbiology, you know the CLSI M100 update is one of the most important annual events on the calendar. Each new edition can mean real changes to how your lab reports susceptibility results, and this year's 36th edition, published in late January, is no exception.
We recently hosted Dr. Shelley Campeau — consultant at Scientific Medical Affairs Consulting, longtime CLSI volunteer, and all-around antimicrobial susceptibility testing (AST) expert — for a deep dive into what's new. Here are the highlights.
Why These Updates Matter (A Real-World Case to Start)
Dr. Campeau opened with a sobering case from Washington University: a patient with recurrent cancer who developed a post-surgical wound infection with a pan-resistant, NDM carbapenemase-producing E. coli, resistant even to cefiderocol.
The point? Carbapenem-resistant organisms, including those carrying metallo-beta-lactamases (MBLs) such as NDM, are becoming harder to ignore even outside endemic regions. And as resistance patterns evolve, so must our testing and reporting tools.
Big News: Aztreonam-Avibactam Breakpoints Are Here
One of the most significant additions in the 36th edition is the inclusion of aztreonam-avibactam (ATM-AVI) breakpoints. This combo drug, FDA-approved in 2025, is a critical option for MBL-producing carbapenem-resistant Enterobacterales (CRE), especially NDM.
Here's why it's such a big deal: metallo-beta-lactamases chew through nearly every beta-lactam except aztreonam — but aztreonam alone often fails because co-existing beta-lactamases degrade it before it can act. Avibactam blocks those co-existing enzymes, leaving aztreonam free to do its job against the MBL.
Previously, labs were improvising with a ceftazidime-avibactam + aztreonam combination while the dedicated drug went through trials. Now, with an FDA-cleared drug and CLSI breakpoints, labs finally have a clear, validated path forward. Even better, the FDA reviewed and harmonized their breakpoints with CLSI's, always a welcome alignment.
One practical note: for disk diffusion, a confirmatory MIC is recommended when isolates fall in the intermediate zone, due to minor error concerns with certain isolates. Worth building into your workflow.
Acinetobacter Updates: Aminoglycosides and More
It had been decades since aminoglycoside breakpoints for Acinetobacter baumannii were reviewed. In some cases, the disk correlated data were from the 1980s. This year, CLSI finally updated them.
The short version: breakpoints were lowered by one dilution for amikacin, gentamicin, and tobramycin, based on updated PKPD and epidemiological cutoff data. The disk zones were also realigned accordingly.
Two other Acinetobacter notes worth flagging:
Tetracyclines: Doxycycline and tetracycline were permanently removed and archived in this edition (they were temporarily pulled in the 35th). A helpful new comment now allows labs to infer minocycline susceptibility if doxy MIC is ≤1 or tetracycline MIC is ≤4, but importantly, you cannot infer non-susceptibility the other way. If doxy or tetracycline tests above those thresholds, minocycline must be tested directly.
Sulbactam-durlobactam: This drug was moved up in Table 1A2, now cascading off the carbapenems rather than being buried in Tier 3, reflecting its importance as a treatment option specifically for carbapenem-resistant infections.
Cefiderocol Disk Diffusion for Stenotrophomonas: A Compromise
The cefiderocol disk breakpoint for Stenotrophomonas maltophilia was reassessed this year after the FDA proposed a higher breakpoint (≥18 mm) than what CLSI had set (≥15 mm). There was legitimate concern about isolates clustering right at the breakpoint, especially given the known variability in cefiderocol disk testing and its media dependence.
The result: CLSI and FDA met in the middle and are now aligned. If you're testing cefiderocol against Steno, check that your current disk breakpoint reflects the updated value.
Neisseria gonorrhoeae Gets a Full SIR Breakpoint Update
Gonorrhea breakpoints don't change often, but this year brought a meaningful update for ceftriaxone and cefixime, now with a complete susceptible/intermediate/resistant (SIR) framework rather than susceptible-only categories. This is important for capturing emerging resistance and linking to clinical failure signals.
Disk diffusion breakpoints for these drugs are expected to follow in the next edition, as more cephalosporin-resistant isolates are needed to complete that data set.
New Beta-Hemolytic Strep Breakpoints for Trimethoprim-Sulfamethoxazole
This one might surprise you — trim-sulfa breakpoints for beta-hemolytic streptococci are now included in the M100. The clinical rationale: clinicians use trim-sulfa in combination with beta-lactams for skin and soft tissue infections that may involve both beta-hemolytic strep and MRSA.
A key nuance here is method: much of the historical data suggesting high trim-sulfa resistance in these organisms was generated using sheep's blood media (which contains thymidine, creating a bypass pathway that falsely elevates MICs). The new breakpoints are validated with appropriate media, and the accompanying comment restricts reporting to SSSI isolates, not pharyngitis, UTIs, or bloodstream infections where clinical data is lacking.
If your lab gets trim-sulfa requests on beta-hemolytic strep, this update is directly relevant to how you report those results.
Burkholderia cepacia Complex: ECVs Removed
In a notable move, the epidemiological cutoff values (ECVs) for Burkholderia cepacia complex were removed from the Appendix. Why? After all clinical breakpoints and disk diffusion interpretations were pulled in prior editions due to poor performance, there was concern that ECVs — which are not meant for clinical use — were being applied that way anyway. Given that the underlying data was also limited and largely extrapolated from other organisms, the decision was made to remove them rather than risk misuse. More educational guidance from CLSI is expected on how to handle this complex clinically.
Appendix D Anaerobe Antibiogram: A Major Overhaul
If you haven't looked at the anaerobe cumulative antibiogram in M100 recently, or ever, now's a great time to revisit it. This appendix received a comprehensive update using contemporary data (through December 2024) from three major reference laboratories, with isolates from around the world tested by the reference agar dilution method.
Key improvements include species-level data where previously only genus-level data existed, more drugs, and a broader range of organisms. This resource can be invaluable for guiding which anaerobic drugs to even offer on panels, supporting empiric therapy while awaiting send-out results, and informing your lab's reporting strategy.
Poll results during the webinar showed that a significant portion of attendees didn't even know this resource existed in M100, so consider this your invitation to dig in.
Direct from Positive Blood Culture Disk Diffusion Updates
A few tweaks to this method this year:
- Piperacillin-tazobactam: A resistant-only breakpoint was added for Enterobacterales. Susceptible-zone results should be confirmed by MIC or standard disk diffusion.
- Cefepime: Previously listed under a different category, now aligned with standard disk diffusion breakpoints.
- Tobramycin for Acinetobacter: Temporarily removed pending harmonization with the updated standard breakpoints, expected to return in the next edition.
QC Range Updates and What They Signal
QC range updates can seem routine, but Dr. Campeau made an insightful point: they're often an early indicator of what's coming to market. Manufacturers often establish QC ranges during clinical trials, well before FDA clearance. New cefiderocol QC guidance also flags a media-related variability issue; if your QC results are trending elevated, your clinical isolate results may be, too.
Implementation Resources: Don't Reinvent the Wheel
Implementing breakpoint updates is a compliance requirement (CAP checklist), but CLSI's implementation toolkit makes it more manageable. It includes educational materials, data collection spreadsheets for verification, and links to CDC/FDA error bank isolate sets. It's updated annually alongside M100, so the aminoglycoside changes we discussed are already reflected. If you're not using it, it's worth a look.
Additionally, updates to M52 (verification of commercial AST systems) and the new M68 document (validation for LDT/modified methods) are expected this summer. If those areas affect your lab, keep an eye out.
What's Coming Next
Dr. Campeau gave a brief preview of topics under CLSI evaluation, including:
- More breakpoints for oral cephalosporins and Enterobacterales
- Acinetobacter additions
- Disk diffusion breakpoints for N. gonorrhoeae (the ones not yet set this cycle)
- Updated M45 document (expected 2027)
Keep Learning and Earn CE While You're at It
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