Performance of disk diffusion and MIC gradient tests in tigecycline susceptibility testing of enterococci: a Nordic multicentre study

Purpose: Evaluating the performance of the European Committee on Antimicrobial Susceptibility Testing (EUCAST) disk diffusion (DD) and minimum inhibitory concentration (MIC) gradient methods for tigecycline susceptibility testing of enterococci using broth microdilution (BMD) as the reference method. Methods: All 83 Nordic clinical microbiology laboratories were invited to test a blinded collection of diverse whole genome sequenced Enterococcus faecalis (n = 10) and Enterococcus faecium (n = 10) strains using the EUCAST DD and gradient MIC methods (Etest or MTS). Results were compared to reference BMD MICs and genotype (presence/absence of known tigecycline resistance determinants). Categorical agreement (CA), essential agreement (EA), and bias were calculated according to international standards. Results: Forty-four laboratories contributed to 880 EUCAST DD and 800 MIC gradient test observations. The EUCAST DD method demonstrated a CA of 97.3% when compared to reference MIC and 93.0% when compared to genotype. Gradient tests achieved an overall EA of 95.9% (Etest: 96.3%; MTS: 95.5%) and a CA of 96.3% when compared to reference MICs and 96.5% when compared to genotype. Both Etest and MTS exhibited acceptable bias (Etest: +21.8%; MTS: -19.1%). Conclusion: The EUCAST DD method and gradient tests demonstrated acceptable performance in accordance with international standards. Errors were mainly related to borderline strains near the tigecycline breakpoint. Further research is required on tigecycline-resistant enterococci and the development of improved AST methods for categorisation of borderline strains.