Euro-GASP external quality assessment scheme for Neisseria gonorrhoeae antimicrobial susceptibility testing
External quality assessment (EQA) is an essential part of any laboratory-based surveillance system, allowing for the monitoring of performance and comparability of results from participating laboratories, 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. This EQA scheme has so far shown high levels of interlaboratory comparability in the presence of differing methodologies. Problems identified previously included reduced comparability of results determined using discs diffusion compared with those determined by agar dilution and MIC gradient strip tests, media not suitably supporting gonococcal growth, and the use of MIC gradient strip tests from one manufacturer.
Materials and methods
The EQA specimen panel was selected by Public Health England (PHE) and distributed by the United Kingdom National External Quality Assessment Service (UK NEQAS). In October 2018, 27 laboratories in 26 participating countries received 10 gonococcal isolates for antimicrobial susceptibility testing. Of the 10 gonococcal isolates provided, one strain was in triplicate, and two strains were in duplicate to test intralaboratory concordance. The remaining isolates were all provided singularly, meaning that the N. gonorrhoeae antimicrobial susceptibility EQA panel comprised six different strains in total. The isolates chosen by PHE were representative of a range of different antimicrobial susceptibility profiles and consisted of the four WHO reference strains, WHO K, O, Q, Z, and two clinical isolates obtained in the UK in 2017. Participating laboratories were requested to test the EQA panel using local methodology (i.e. MIC gradient strip test, agar dilution or disc diffusion) and relevant international breakpoints (i.e. EUCAST, CLSI etc.) against a range of antimicrobial agents. Results were submitted directly to UK NEQAS who issued individual laboratory reports. The results were then supplied to PHE who decoded and analysed the results based on the categories of susceptibility assigned.
Twenty-seven laboratories returned EQA results to UK NEQAS. Most laboratories used MIC gradient strip tests and EUCAST breakpoints. The highest level of categorical agreement (other than spectinomycin; 100%) was seen with ceftriaxone (98.6%), while the lowest was seen with azithromycin (77.6%). Overall concordance decreased for most antimicrobials in comparison with the previous distribution, except for ciprofloxacin, which increased slightly from 94.1% to 98.1%. Overall, 95.2% and 99.4% of the reported minimum inhibitory concentrations (MICs) were within one (essential agreement) and two doubling dilutions of the modal MIC, respectively.
Discussion and conclusion
There has been further harmonisation of susceptibility testing methodologies and breakpoints used by participating laboratories; most laboratories used MIC gradient strip tests and all applied EUCAST breakpoints for interpretation of MIC results. Overall, the laboratories participating in the EQA scheme QA18 performed well and showed good levels of competency in testing N. gonorrhoeae isolates of unknown phenotype. Categorical agreement decreased slightly in this distribution when compared with 2017, with the exception of ciprofloxacin. The inter- and intralaboratory concordance was high in most cases, demonstrating comparability between different testing methodologies and allowing confidence in decentralised testing for surveillance purposes. Most susceptibility category discrepancies were attributable to strains with MICs on or close to a breakpoint, which highlights the need to consider the actual MIC as well as susceptibility category when interpreting susceptibility results. Analysis of the individual results submitted by the participating laboratories highlighted one centre in need of further guidance to help bring them into line with the Euro-GASP1-recommended target of 95% of MICs within two doubling-dilutions (fourfold) of the modal MICs and beta-lactamase assessment.
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