Updates from Jan 2024 CLSI AST Subcommitee and Working Groups

Updates from the Jan 2024 CLSI AST Subcommittee and Working Groups

January 2024 CLSI Committees Week

CLSI committees convene multiple times annually to discuss emerging protocols, evolving practices, and the formulation of novel standards within the realm of laboratory medicine.

Recently, Peter Trabold, Regulatory Affairs Specialist at MDC, had the opportunity to attend the Clinical and Laboratory Standards Institute (CLSI) Antimicrobial Susceptibility Testing (AST) Subcommittee and Working Group Meetings held from January 18-23, 2024.

Here are some notable updates and developments discussed during this past subcommittee meeting.

Antimicrobial susceptibility testing
M22 — QC for Commercially Prepared Media

M22, although still an accepted protocol, is now slated to be archived and will no longer receive updates.


M35 — Abbreviated Identification of Bacteria and Yeast, 2nd Edition

The updated version of M35 is currently in development. Filamentous fungi will be included in the document. This expansion aims to enhance the utility of the guidelines, particularly for laboratories in limited resource countries.


M45 — Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria, 3rd Edition

M45 is currently updated every five years. A new process is in the works to update the document more frequently, potentially as a live document. The next update is slated for spring 2025.


M57 — Principles and Procedures for the Development of Epidemiological Cutoff Values for Antifungal Susceptibility Testing, 1st Edition

New annex tables are being developed, and the document will undergo reorganization and more than 50 Epidemiological Cutoff Values (ECVs) will be added.


M68 (In development) – Protocol for Updating Breakpoints in a Clinical Lab

M68 aims to provide a protocol for updating breakpoints in a clinical laboratory setting. It will be aligned with M52 (Verification of Commercial Microbial Identification and Antimicrobial Susceptibility Testing Systems, 1st Edition), ensuring consistency and compatibility across standards.


M100—Scheduled for Publication in April 2024

The update to M100 is scheduled for publication on February 28th, 2024. The FDA may not acknowledge the breakpoint changes proposed in the M100 update right away. The agency may adopt the updated breakpoints outlined in M100, or alternatively, establish their own breakpoints.


General Comments on M100 Updates

Disk Diffusion Methodology: There was consensus within CLSI that Disk Diffusion is a standard method, albeit not a reference method. This distinction between standardized and reference methodologies prompted discussions aimed at resolving any contradictory language in CLSI documents.

Breakpoint Considerations: CLSI generally avoids introducing breakpoints within the middle of the wild-type distribution due to potential data analysis challenges. Additionally, the differentiation between documents classified as Standards or Guidance, such as M45 (Guidance) and M100 (Standard), was underscored.

AST Manufacturers Collaboration: Noteworthy discussions highlighted the potential for AST manufacturers to collaborate with pharmaceutical companies, thereby obtaining financial assistance for the development and inclusion of antimicrobials into their devices.

Organism-Specific Considerations: Specific considerations were addressed for organisms such as Staphylococcus, with discussions on the use of alternative nomenclature like “Staphylococcus other than S. aureus (SOSA)” and the potential incorporation of the “S. aureus complex,” though a consensus was not reached at this time.


M100 Breakpoint Updates for 2025:

Aztreonam Adjustment: Aztreonam’s reclassification from tier 4 to tier 3 in Table 1A of M100 was discussed, with implications primarily focused on the U.S. jurisdiction. An ex-US table may be developed in the future.

Fluoroquinolones: Fluoroquinolones (moxifloxacin, levofloxacin, and ciprofloxacin) are slated to be removed from the warning limitation for Tables 2A through 2J in M100. Cefazolin will require further deliberation.

Phenotypic Rules for CRE (Carbapenem-resistant Enterobacterales): Discussions on phenotypic rules emphasized the significance of NDM and KPC as markers for CRE-producing strains. A vote for CRE testing of Enterobacterales isolates with specific resistances to Ertapenem, Imipenem, or Meropenem was rejected, and slated for discussion again in June.

eCIM method: This method is used for detecting metallo-beta-lactamase (MBL) alongside mCIM in Enterobacterales and Pseudomonas. Concerns were raised regarding the reliability of eCIM in the presence of both MBL and Serine beta-lactamase (SBL), such as KPC and OXA-48. In response, a proposal was accepted to add limitations to eCIM testing, aiming to address potential inaccuracies.

Fluoroquinolones (moxifloxacin, levofloxacin, and ciprofloxacin): It was decided that Fluoroquinolones will be removed from the warning limitation for Tables 2A through 2J in M100. However, the exclusion of cefazolin from this limitation requires further deliberation.

A. baumannii: The A. baumannii working group announced plans to evaluate new potential breakpoints in their June meeting, emphasizing the importance of molecular testing in determining appropriate therapy.

Burkholderia cepacia complex: Discussions highlighted the removal of Disk Diffusion (DD) breakpoints in June 2023, favoring broth or agar dilution methods due to sensitivities to media differences. There were considerations about potentially transferring Breakpoint Minimum Dilution (BMD) breakpoints to M45 and removing them from M100, although this decision may undergo reevaluation, with changes not expected until 2025.


M45 Breakpoint Updates:

Penicillin and M45: It was noted that certain breakpoints for Penicillin include instances where susceptible breakpoints surpass Epidemiological Cutoff Values (ECVs).

Abiotrophia/Granulicatella, Lactococcus, and Micrococcus: Adjustments were approved with breakpoints set to increase to 2/4/8.

Lactobacillus and Pediococcus: Reductions were approved for Lactobacillus and Pediococcus, with breakpoints lowered to <=2 from 8.

Leuconostoc: Proposals to reduce Leuconostoc breakpoints to <=4 from 8 were also approved.

Tetracycline (tetracycline and doxycycline) and M45: Proposals were made to lower breakpoints for Aeromonas, Bacillus, Campylobacter, Corynebacterium, Leuconostoc, and Vibrio to the Epidemiological Cutoff Value (ECV) for each organism listed on the slide from 4/8/16. However, the discussion highlighted the need for reevaluation as these adjustments might differ from breakpoints for other species under M100.


Quality Control (QC):

Newer strains: It was noted that newer strains exhibit fewer out-of-range results, which is likely attributed to improved procedures for establishing ranges. This trend suggests an enhancement in QC practices and reflects positively on laboratory quality assurance measures. Additionally, it was observed that the majority of errors encountered during QC testing were attributed to system or manufacturing errors, emphasizing the critical importance of stringent quality control measures in laboratory processes.

QC test frequency: Plans are currently underway to reduce the frequency of QC testing. While current recommendations are available in Table 5F, it was acknowledged that adjustments may be necessary. These adjustments could potentially deviate from the guidelines outlined in M100 or be additional to M100. Final decisions regarding the frequency of QC testing and associated protocols are yet to be determined and finalized.

Combination drugs: When integrating combination drugs, such as ceftibuten-avibactam, into QC procedures, it is essential to ensure that the two routine QC strains (1R and 1S) do not exhibit significant overlap in ranges. This precautionary measure is crucial for maintaining accuracy in QC assessments.

Breakpoint Minimum Dilution (BMD) testing: Two methods are utilized to establish the range. This includes a 3 or 4 dilution range, with a recommended 60% shoulder for a 4-dilution range CLSI method, which is considered the preferred approach. The shoulder is calculated from the main peak as a percentage. Additionally, the RangeFinder method offers an alternative means of determining range. The RangeFinder is available from CLSI.

Disk Diffusion (DD) testing: Two methods are employed to establish the range. This includes the Gavan Statistic, considered the preferred method, and the RangeFinder, which provides an additional means of range determination. It is emphasized that efforts should be made to maintain ranges within 9 mm or less to ensure optimal accuracy and consistency in DD testing results.


Antifungal Working Group:

Aspergillus fumigatus: Voriconazole, Isavuconazole, and Posaconazole were identified as substances nearing submission of pre-Investigational New Drug Exemption (pre-IDE) requests to the FDA for establishing breakpoints against Aspergillus fumigatus.

Rezafungin: An antifungal agent product developed by Melinta Therapeutics recently received FDA approval in March 2023. The group noted once-weekly treatment and slow clearance rates. However, while Breakpoint Minimum Dilution (BP) values have been established for Candida species, the focus remains primarily on susceptibility. Challenges persist in determining resistance accurately. Nevertheless, the CLSI’s vote to reaffirm tentative breakpoints and finalize them reflects progress in this area. Plans are underway to compile a rational document for submission to the FDA, advocating for the establishment of breakpoints based on these findings.


Overall — Continuous Collaboration

Throughout the sessions discussed, one message was clear: continuous collaboration is paramount for progress. The commitment within the medical and laboratory communities to work together continued to be a driving force during the January CLSI Committees Week —a shared commitment to ensuring the highest standards of quality and reliability in laboratory testing and clinical practice.

More CLSI resources:
AST Meeting Files and Resources

This summary is not intended as a comprehensive list of meeting minutes. Readers are encouraged to refer to source material and official communication from clsi.org.

This web site is provided for general awareness and informational purposes only and does not constitute providing medical advice or professional services.

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