Human choriogonadotropin (hCG): Comparisons between determinations of intact hCG, free hCG β-subunit, and "total" hCG + β in serum during the first half of high-risk pregnancy

Abstract

We have studied the concentrations of intact human choriogonadotropin (hCG) and the free hCG β-subunit in blood samples from singleton pregnancies at risk for habitual or threatened abortion. The samples were obtained weekly between the 6th and 12th weeks and in the 14th and 16th weeks of gestational age. The concentrations of intact hCG, of the free hCG β-subunit, and of "total" hCG (i.e., intact hCG and the free hCG β-subunit: hCG + β) were measured in serum by specific immunoassays. The distributional statistics (the 5th, 50th, and 95th percentiles) of "total" hCG + β and of intact hCG showed very similar patterns, whereas the response curves for the free hCG β-subunit showed very much lower serum concentrations. From these data we also estimated distributional statistics of the percent molar ratios of free hCG β-subunit to intact hCG. We conclude that (a) the relatively small proportion of free hCG β-subunit in serum during the first half of singleton pregnancy is far too low to Interfere with the applied "total" hCG assay, as compared with the serum values obtained for intact hCG, and (b) the percent molar ratios of free hCG β-subunit to intact hCG, or to "total" hCG + β, never exceeded 1.0% throughout the period of pregnancy studied.