Comparative evaluation of broth microdilution and gradient diffusion methods in Burkholderia cepacia complex susceptibility testing: implications of transitioning to ECV-based interpretation

Evaluate the performance of broth microdilution (BMD) and gradient diffusion (Etest) methods in anti-microbial susceptibility testing (AST) of Burkholderia cepacia complex (BCC) isolates, with a particular focus on the impact of transitioning from CLSI MIC breakpoints to epidemiological cut-off values (ECVs) in the 2025 guidelines. A total of 90 clinical BCC isolates were collected and identified to the species-level using MALDI-TOF mass spectrometry. AST was performed using BMD and Etest for minocycline, meropenem, trimethoprim-sulfamethoxazole, ceftazidime and levofloxacin. Results were interpreted using 2024 MIC breakpoints and re-analysed with 2025 ECVs. Categorical agreement (CA), essential agreement (EA) and classification errors (VME, ME, MI, WT-ME and NWT-VME) were calculated. In addition, species-stratified analysis was also conducted and medical records of 44 patients were reviewed to assess the correlation between in vitro susceptibility and clinical outcomes. Under CLSI 2024 MIC breakpoints, CA between Etest and BMD varied, highest for trimethoprim-sulfamethoxazole (94.4%) and lowest for meropenem (12.2%). EA for all agents was below the 90% threshold. Using CLSI 2025 ECVs, CA improved for all agents except trimethoprim-sulfamethoxazole, but error rates remained unacceptable for all except minocycline. Species-specific resistance patterns were observed. Among 44 clinical cases, only 40.9% showed concordance between in vitro susceptibility and treatment outcomes. Although the adoption of ECVs improved CA for some agents, MIC-level discrepancies between Etest and BMD persisted. Species-specific resistance patterns and the low concordance with clinical outcomes (40.9%) highlight the limitations of AST alone in guiding BCC treatment decisions.