Each year the laboratories participating in the project were requested to fill in excel-file templates with national summary data on the occurrence of antimicrobial resistance from different bacterial species and groups. The participants were asked to submit national data but if this was not possible they were advised to submit regional or institutional data as done by England, including Wales. The data for some countries were incomplete e.g. for Austria and Latvia and data was deducted for others if the number of isolates were lower than 31.

Invitation was announced annually through the network by email or facsimile to all ARBAO members to participate in self-evaluating proficiency tests (EQAS external quality assurance system) for antimicrobial susceptibility testing. The tests were conducted each year to test if the current methodologies were accurate, adequate and reliable 12. In addition, the EQAS served as a tool to pin out the laboratories from where annual data were reliable. The goal was to have all laboratories to perform antimicrobial susceptibility testing with a maximum of 10% total deviations (minor, major, or very major deviations) and a maximum of 5% critical deviations (major or very major deviations).

Eight strains of each of the species Staphylococcus aureus, Mannheimia haemolytica, Pasteurella multocida, Streptococcus dysgalactiae, Streptococcus uberis and Escherichia coli were selected for each EQAS iteration. Strains were obtained from the bacterial strain collection at the National Food Institute, Technical University of Denmark. All strains were included in only one EQAS iteration. The strains were inoculated to agar stab cultures for shipping to the participating laboratories. Participating laboratories also received a lyophilised reference strain as a quality control strain for susceptibility testing (E. coli ATCC 25922; Campylobacter jejuni ATCC 33560; Enterococcus faecalis ATCC 29212, S. aureus ATCC 25923, and S. aureus ATCC 29213) in each EQAS shipment.

The laboratories were instructed to follow specified testing instructions; subculture the test strains and propagates the quality control strains prior to performing the susceptibility method routinely used by the laboratory. In addition, laboratories were advised to maintain the quality control strain for future proficiency tests. After completion of the susceptibility testing of the test strains and the quality control strain, the participating laboratories were instructed to record the obtained results using MIC values or zone-diameter in millimetres and categorize each of the tested strains as either "resistant" (R), "intermediate" (I) or "susceptible" (S) against each tested antimicrobial agent using the breakpoints routinely used in their laboratory. They were then asked to record the information on the participating laboratory record sheet.

After submitting results, participating laboratories received an individual report. The individual reports for the participating laboratories reported all deviations from the expected values and suggestions of how to either solve or investigate the problem. For the quality control strains, deviations were defined as values that exceeded the quality control range of the strain. Deviations of the antimicrobial susceptibility results were categorised as minor, major or very major. A minor deviation was defined as an intermediate strain that was classified as susceptible or resistant or vice versa (i.e. I ↔ S or I ↔ R). A major deviation was defined as a susceptible strain that was classified as resistant (i.e. S → R). A very major deviation was defined as a resistant strain that was classified as susceptible (i.e. R → S).

The overall performance and the results were between 85–100% correct. In general, the concordance between results for Gram-negative bacteria was good e.g. 93% in 2003 and 95% in 2004 for E. coli. Therefore the summary results for these species might be compared between countries generally.

Each laboratory reported annually, which methods were used for antimicrobial susceptibility testing. Laboratories in Austria, England (including Wales), France, Italy, Latvia and Poland used disc diffusion test, whereas Denmark, The Netherlands, Norway, Sweden and Switzerland used the broth micro dilution method by which minimal inhibition concentration (MIC) are determined. Spain and Finland used both MIC determination and disc diffusion, whereas Belgium used tablet diffusion. Portugal utilized a micro-broth dilution test – the automated ATB susceptibility test strip http://www.biomerieux-diagnostics.com. The manufactures of discs and microtitre plates for broth microdilution differed between countries.

All countries reported the extent to which they followed the standards of the Clinical and Laboratory Standards Institute (CLSI), M31-A2, M7-A6 or M2-A8 3. Austria, England (including Wales), France, The Netherlands, Norway, Sweden and Switzerland reported that they did not use CLSI breakpoints as listed in the fifteenth international supplement M100-S15 to determine the antimicrobial susceptibility levels but a mixture of these and national established breakpoints. Despite the differences in used breakpoints all of the countries performed susceptibility testing within the EQAS QC threshold.