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 (Sep. 1990), 257-258. DOI:https://doi.org/10.3109/03009739009178599.