Human chorionic gonadotropin: properties and assay methods.

Modern HCG assay methodology provides a level of sensitivity such that "HCG" immunoactivity in normal individuals or other unexplained low-level "HCG" measurements may be problematic. Two-site assays improve specificity for authentic HCG by selectively measuring intact hormone, and such measures correlate well with biological activity. In some cases, 125I has been replaced with nonradioisotopic labels without sacrificing sensitivity. On the negative side, such two-site assays will not reveal the presence of free subunit that may originate from trophoblastic tissue. Because free beta subunit appears in the sera of patients with GTD, monitoring intact HCG alone may pose a risk. Thus, some form of subunit monitoring by total-beta or free-beta assay is desirable. If the beta subunit/HCG ratio reflects the degree of trophoblastic cell differentiation and aggressiveness, a combination of specific whole molecule and free subunit measurements may provide the ideal monitoring for GTD.