Quality control schemes are a practical tool used in clinical laboratories and an essential element for any quality assurance process. In France, external quality assessment schemes (EQAS) can be mandatory (as national quality control organized by AFSSAPS) or voluntary as those suggested by French associations (ProBioQual, CTCB or Asqualab). These EQAS usually evaluate participants according to their performances: this ranking depends on acceptability limits which are here compared. Various examples based on ProBioQual's background illustrate difficulties to plan out analytical quality specifications. A comment is given about the best criteria (state of the art or biological variation mainly) to be considered to delimit analytic goals. This discussion includes approaches suggested by French committee on accreditation (Cofrac). All criteria could be criticized but it is important to compare oneself laboratory to peers and also to take account of biological variation.