Comparability of antimicrobial susceptibility test results from 22 European countries and Israel: an external quality assurance exercise of the European Antimicrobial Resistance Surveillance System (EARSS) in collaboration with the United Kingdom National External Quality Assurance Scheme (UK NEQAS).

The goal of this exercise was to organize external quality assurance (QA) of antibiotic susceptibility testing for laboratories participating in EARSS and to assess the comparability of susceptibility test results across countries, and guidelines. In September 2000, UK NEQAS distributed a set of three Streptococcus pneumoniae strains, two Staphylococcus aureus strains and one Streptococcus haemolyticus strain. Laboratories reported the guideline followed, the interpretation of the susceptibility test result and the MIC, if tested. In this study we considered results 'concordant' if the reported interpretation of the participating laboratory agreed with the designated interpretation of reference laboratories. Overall, 433 (92%) of 471 laboratories from 23 countries reported back. Of the 8685 tests that were assessed, 8322 (96%) were interpreted correctly by the participants. Concordance for detection of penicillin non-susceptibility in the three S. pneumoniae strains was 96%, 90% and 87%, respectively. Laboratories performed extremely well in detecting oxacillin resistance in the homogeneously methicillin-resistant S. aureus (MRSA) strain, but the concordance rate dropped from 100% to 77% in the heterogeneously resistant MRSA strain. Concordance for detection of teicoplanin resistance in the S. haemolyticus strain was 82%. We stratified concordance rates first for country and then for guideline used, but observed only minor differences among countries and guidelines. Quantitative methods yielding an MIC were more concordant than non-MIC methods for penicillin resistance in the S. pneumoniae strains (94% versus 79%). The NCCLS guideline was the most frequently followed, by 61% of laboratories from 19 countries. This exercise shows that, overall, countries participating in EARSS are capable of delivering susceptibility data of good quality. The comparability of susceptibility data for penicillin resistance in S. pneumoniae and for homogeneous methicillin resistance in S. aureus is satisfactory among European countries and across guidelines. However, we emphasize the importance of determining an MIC for suspected penicillin non-susceptible S. pneumoniae and for suspected glycopeptide non-susceptible S. aureus. Laboratories, particularly in some countries, may need to improve their capability to detect oxacillin resistance in heterogeneously resistant MRSA. For continuous external quality assessment we recommend that laboratories participate in national and international schemes with frequent distribution of control strains.