Penttilä I. M., Ryynänen M., Hämäläinen E., Puhakainen E., and Savolainen K., “6.1.3.2 Quality Specifications for S-AFP and S-HCG-β Determinations for Screening of Fetal Abnormalities”, ujms, vol. 98, no. 3, pp. 375-380, Sep. 1993.