Conventional true values compared

According to ISO/CD 18113-1:2005, the conventional true value of a control material can be an assigned value (obtained with a primary or reference measurement procedure), a consensus value or a procedure-defined value. The present study demonstrates that, for some biological quantities, the overall consensus value (“trimmed” mean value”) obtained in a proficiency testing program, which includes all results independently of the method of measurement, is not acceptable as a surrogate for a primary or reference method value. Therefore, for the biological quantities taken into account in this study, overall consensus value is not as good as primary or reference method value to estimate systematic error.