Sensitive Enzyme and Erythro Immunoassays for Detection of Human Pregnancy

Abstract

Human chorionic gonadotropin (hCG) is composed of two non-covalently linked dissimilar glycosylated α and β subunits. The α subunit of hCG is virtually identical to the α subunits of three pituitary gonadotropins, namely hLH, hFSH and hTSH. Biological and immunological specificity of the molecule thus reside in the β subunit. It is normally a product of trophoblast and detectable in the circulation within 8 to 10 days post-fertilization (Catt et al., 1975; Saxena et al., 1974). Studies on human embryos cultured in vitro demonstrated that the synthesis of hormone starts as early as 170 h after fertilization, e.g. in preimplantation embryos (Fishel et al., 1984). Its level, during the first week after the expected menstrual period, i.e. third week of pregnancy, is around 0.3–0.4 IU/ml of blood (Braunstein et al., 1976) with a doubling time of 1.7–2 days in early gestation (Braunstein et al., 1973). It has traditionally been used as an index of pregnancy and is the basis of all available bioassays and immunoassays.