Multicentric evaluation of the MTS-SAS® for reliable antibiotic synergy testing in clinical laboratories

Objectives: The rise of antimicrobial resistance poses a major challenge for both clinicians and clinical microbiologists. There is an increasing need for user-friendly and reliable methods to assess the activity of antibiotics against multidrug-resistant (MDR) strains. Although synergy testing provides valuable insights, conventional methods such as checkerboard assays and time-kill studies are labor-intensive, technically demanding, and difficult to standardize. This study evaluated the MTS-SAS® (MIC Test Strip - Synergy Application System, Liofilchem®, Italy), a commercial gradient diffusion assay developed for antibiotic synergy testing. Methods: The performance of MTS-SAS® was evaluated in comparison with the checkerboard microdilution method, used as the reference standard. Nine antibiotic combinations were tested against ten different bacterial strains across 11 Italian hospitals. Inter-laboratory reproducibility and agreement with the reference method were analyzed. Results: The concordance between MIC test strips and the broth microdilution (BMD) method was 98.4%, with 1.6% showing discordant results - all within a three-dilution range. Among 996 synergy determinations, MTS-SAS® demonstrated high reproducibility across all centers (96.7%), while only 3.3% of tests showed discordant synergy classifications (e.g., synergy vs. indifference). Comparison with the checkerboard method demonstrated an overall concordance of 96.2%, despite the absence of specific operator training at each site. Conclusion: These findings support MTS-SAS® as a practical and reliable alternative to conventional synergy testing methods, particularly suitable for routine clinical settings and laboratories lacking advanced microbiological expertise.