External quality assessment of antimicrobial susceptibility testing in Europe.

Comparability of results of antimicrobial susceptibility testing is essential for resistance surveillance studies. As different methods may be used in different countries, there may be particular problems with international comparisons of resistance rates. Data from external quality assessment (EQA) surveys participated in by laboratories from several European countries allow comparison of performance between countries. In this study, success rates with organism-antimicrobial agent combinations known to be difficult to test were examined. With penicillin resistance in pneumococci; vancomycin and high-level gentamicin resistance in enterococci; ampicillin, co-amoxiclav and chloramphenicol resistance in Haemophilus influenzae and methicillin resistance in staphylococci there were differences between countries in success rates for discrimination of resistant strains. This study suggests that differences between countries in rates of resistance for some organism-antimicrobial agent combinations should be interpreted with caution. International EQA is useful in the demonstration and clarification of such differences.