A testing time for gradient strips: an evaluation of ETEST for the antimicrobial susceptibility testing of Neisseria gonorrhoeae

Introduction. Antimicrobial susceptibility testing (AST) of Neisseria gonorrhoeae isolates is recommended in the UK to ensure antimicrobial stewardship and detection of multi-drug and extensively resistant cases. In diagnostic and reference laboratories, this testing is primarily carried out via a gradient strip. However, agar dilution methodology may also be used for highthroughput testing. Gap statement. N. gonorrhoeae is not a validated species on all ETEST (bioMérieux, France) gradient strip formulations available, and, therefore, additional comparative validation data are required to support use in clinical laboratory settings. Aim. To determine the reproducibility of ETEST for AST of N. gonorrhoeae, and to demonstrate the comparability of susceptibility results obtained using agar dilution and gradient strip methods. Methodology. Modal ETEST MICs for six well-characterized World Health Organization N. gonorrhoeae control strains, against eight antimicrobials, were calculated. ETEST modal MICs were compared to published and local, historical agar dilution MICs. MICs were interpreted using European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints. Concordance of ETEST modal MICs, within essential and categorical agreement, for each strain was calculated. Results. Overall, 95.80% of modal ETEST MICs were within essential agreement with published MICs. Where variance from exact concordance was noted, a systematic shift was observed to lower MICs in this study. On three occasions, variance from the published MIC resulted in a categorical classification change. On two occasions, the modal ETEST MIC in this study was two doubling dilutions lower than the published MIC. Neither resulted in a categorical classification change. When modal ETEST and agar dilution MICs were compared, overall essential agreement was 83.30%. However, this increased to 94.4% when concordance was analysed for clinically important antimicrobials: azithromycin, cefixime and ceftriaxone. Again, a systematic shift to lower MICs for ETEST was observed in this study. Categorical agreement was 83.3% for all antimicrobials and 100% for clinically important antimicrobials. Conclusion. Excellent concordance was demonstrated for MICs generated using ETEST with published MICs. Good concordance was observed for MICs generated using two different susceptibility testing methodologies. Where variance was noted, MICs generally read lower on ETEST in this study. ETEST remains fit for purpose for the AST of N. gonorrhoeae, a clinically important pathogen.