Ruokonen , A., Hyltoft-Petersen , P. and Blaabjerg , O. (1990) “External Quality Assurance Model for Improvement of Specificity and Assuracy of Serum Hormone Determinations in the Nordic Countries”, Upsala Journal of Medical Sciences, 95(3), pp. 257-258. doi: 10.3109/03009739009178599.