Euro-GASP external quality assessment scheme for Neisseria gonorrhoeae antimicrobial susceptibility testing – 2025

Introduction

External quality assessment (EQA) is an essential component of any laboratory-based surveillance system, enabling the monitoring of laboratory performance and comparability of results across participating laboratories, as well as the identification of potential issues and deployment of resources and training where necessary. An EQA scheme for antimicrobial susceptibility testing in Neisseria gonorrhoeae has been available to laboratories participating in ECDC’s European Sexually Transmitted Infections (STI) surveillance network since 2010. Over time, this EQA scheme has demonstrated high levels of inter-laboratory comparability, despite the use of differing methodologies.

Materials and methods

The EQA specimen panel of 10 gonococcal isolates was selected by the UK Health Security Agency (UKHSA) and Örebro University Hospital (ÖUH) and was distributed by the United Kingdom National External Quality Assessment Service (UK NEQAS). Of the 10 gonococcal isolates provided, one strain was in triplicate and two strains were in duplicate to test intra-laboratory concordance. The remaining isolates were provided singly, such that the

N. gonorrhoeae antimicrobial susceptibility EQA panel comprised six different strains. The strains were representative of a range of different antimicrobial susceptibility profiles and consisted of four WHO reference strains (WHO S2, WHO K, WHO O, and WHO F) and two clinical strains (H25-549 and H25-327). Laboratories were asked to test the EQA panel using local methodology (i.e. minimum inhibitory concentration (MIC) gradient strip test or agar dilution) and relevant breakpoints (i.e. EUCAST, CLSI, etc.). Antimicrobial agents tested included azithromycin, cefixime, ceftriaxone, ciprofloxacin, gentamicin, spectinomycin and tetracycline. Strains were also tested for beta-lactamase production. Results were submitted directly to UK NEQAS, who issued individual laboratory reports. The raw results were supplied to UKHSA and ÖUH, who decoded and analysed them. Antimicrobial susceptibility category concordance (categorical agreement) was assessed using the consensus category (most often reported category) of susceptibility for each tested strain. MIC concordance was assessed by examining MIC results within one (essential agreement) and two doubling dilutions of the modal MIC. Intra-laboratory concordance was examined using the triplicate and the two duplicate strains.

Results

In September 2025, the EQA panel was dispatched to 28 laboratories in 28 European Union/European Economic Area (EU/EEA) countries, 27 (96.4%) of which returned results to UK NEQAS. All laboratories used MIC gradient strip tests for one or more antimicrobials, and all stated that they used EUCAST breakpoints.

Overall categorical agreement ranged from 92.8% (tetracycline) to 100% (ciprofloxacin), exceeding the recommended 90% threshold for all antimicrobials. Compared with the 2024 EQA, categorical agreement increased for azithromycin (94.4% to 95.7%) and ciprofloxacin (99.4% to 100%), but decreased for cefixime (100% to

94.2%), ceftriaxone (99.2% to 94.7%), spectinomycin (100% to 98.8%), beta-lactamase detection (98.2% to 96.7%), and tetracycline (94.8% to 92.8%). Most susceptibility category discrepancies occurred in strains with MICs close to a resistance breakpoint. Overall, 95.9% and 98.6% of MICs were within one (essential agreement) and two doubling dilutions of the modal MIC, respectively, corresponding to a significant increase in essential agreement compared with 2024 (93.3%, p<0.001). Essential agreement for individual antimicrobials ranged from 90.7% (ciprofloxacin) to 99.5% (gentamicin). Compared with 2024, essential agreement increased for all antimicrobials except cefixime and ciprofloxacin; the increases were statistically significant for azithromycin (89.5% in 2024 to 95.4% in 2025, p=0.01) and tetracycline (92.1% in 2024 to 96.5% in 2025, p=0.04).

At the individual laboratory level, the average inter-laboratory categorical concordance score for the core antimicrobials and beta-lactamase decreased from 99.2% in 2024 to 96.3% in 2025. However, 19/27 (70.4%) laboratories scored 95% or higher and 11/27 (40.7%) achieved a perfect score of 100%. The average inter-laboratory MIC concordance score for the core antimicrobials decreased slightly from 90.8% in 2024 to 89.2% in 2025. Four laboratories reported >5% of MIC results greater than two doubling dilutions from the modal MIC across the core antimicrobials. Intra-laboratory MIC concordance remained stable and high (96.2% in 2024 and 96.3% in 2025), with 21/27 (77.8%) laboratories scoring 95% or higher, including three laboratories with a perfect score of 100%.

Discussion and conclusion