From the Research
The Clinical and Laboratory Standards Institute (CLSI) M-100 guidelines for 2025 should be followed for laboratory diagnosis, as they provide standardized methods for antimicrobial susceptibility testing and interpretive criteria for clinical laboratories. These guidelines include detailed protocols for specimen collection, processing, testing methodologies, quality control procedures, and interpretation of results. For laboratory diagnosis, laboratories should follow the current edition's specific breakpoints for determining whether organisms are susceptible, intermediate, or resistant to antimicrobial agents. The guidelines recommend appropriate testing methods based on organism type, including disk diffusion, broth microdilution, and automated systems.
Key Recommendations
- For critical infections, rapid diagnostic methods should be employed when available, as seen in the evaluation of early reading of broth microdilution technique for polymyxin B, which presented 93.2% of essential agreement and 97.9% of categorical agreement with the standard reading of BMD 1.
- Quality control strains specified in the guidelines must be tested regularly to ensure accuracy of results.
- Laboratories should report results with interpretive categories and, when appropriate, include comments about intrinsic resistance patterns or detection of specific resistance mechanisms.
- The guidelines also provide recommendations for confirmatory testing of certain resistance mechanisms such as carbapenemases, extended-spectrum beta-lactamases (ESBLs), and methicillin resistance.
Testing Methods
- Disk diffusion, broth microdilution, and automated systems are recommended testing methods based on organism type.
- The performance of the MICRONAUT-S Anaerobes MIC plate, a commercially available broth microdilution method, showed a high concordance with the gradient diffusion method, with overall essential and categorical agreements resulting >95% and >97%, respectively 2.
- Etest gradient diffusion represents a valid option for antimicrobial susceptibility testing of B. pseudomallei against amoxicillin/clavulanate, ceftazidime, doxycycline, and imipenem, with ≥97% category interpretation and ≥93% essential agreement of MIC 3.
Implementation
Implementation of these standardized methods ensures consistency across laboratories and helps clinicians make appropriate treatment decisions based on reliable antimicrobial susceptibility data, ultimately improving patient outcomes through targeted antimicrobial therapy. The CLSI guidelines provide a comprehensive framework for laboratory diagnosis, and adherence to these guidelines is crucial for ensuring accurate and reliable results. As noted in the overview of changes to the CLSI Performance Standards for Antimicrobial Susceptibility Testing, M100, 31st Edition, the guidelines include new and revised breakpoints and testing recommendations 4.