Assays of antibodies to a C‐terminal peptide or the entire β‐subunit of human chorionic gonadotropin

Antibodies were assayed in serum samples obtained from rabbits or women immunized with a vaccine based on a C-terminal peptide (109-145; CTP) of the beta-subunit of human chorionic gonadotrophin (hCG) with use of a ligand-binding assay. In rabbit samples, two types of assay were used. The first "homologous" type was based on CTP as tracer and standard. In the second "heterologous" type, directly reflecting the hCG-neutralizing potency, hCG was used as tracer and standard. The equilibrium constants of antibodies were substantially higher in the homologous than in the heterologous assay, indicating that the fit of hCG to the antibodies was worse than that of CTP. This was further confirmed by very low cross-reaction values of hCG. In addition, hooks occurred in Scatchard plots when the heterologous assay type was used, both with rabbit and human samples. However, a high correlation between the results of the homologous and heterologous assay was observed (r = 0.97; P < 0.05). Therefore, it is envisaged that the possibility of using the homologous, analytically less complex assay will be further investigated in future clinical studies. Antibodies raised in women to the beta-subunit of hCG had equilibrium constants higher by one to two orders of magnitude than those of the anti-CTP antibodies. The present definition of a threshold pregnancy-preventing level of antibodies disregards their avidity. It is suggested that in future studies, the problem of varying avidity could be solved by individually adjusting the threshold levels with respect to antibody avidity.