In the Nordic Protein Project an external control scheme (external quality assessment) was combined with the two other indispensable aspects of analytical quality, i.e. standardization (with a common high quality calibrator) and specification of needed analytical quality for sharing common reference intervals for nine serum proteins in the Nordic countries. The quality specifications are reliable for the purpose and given in clinical chemical terms--ready for application to the control systems. Further, a control design for disclosing external and internal errors, separately, is designed with respect to calibration and robustness towards analytical interference from turbid patient samples.