External quality assessment: currently used criteria for evaluating performance in European countries, and criteria for future harmonization.

A questionnaire was circulated to European countries seeking information on the criteria used for acceptable performance in external quality assessment schemes. Responses were obtained from 21 countries. Fixed limits are used in 13 countries but the basis for these varies widely and includes clinical decision making, biological variation, views of experts, the state-of-the-art, and combinations of approaches. Variable limits based upon statistical analysis of the performance attained are used in 8 countries. The many advantages of harmonization in Europe have prompted the development of criteria based upon within- and between-subject biological variation for use in schemes which circulate single specimen challenges. Currently used criteria, which show much diversity, are compared with these proposals, and the empirical nature of the majority of the former is demonstrated.