The HCG-beta-subunit radioimmunoassay: potential error in HCG measurement related to choice of labeled antigen.

Antiserum generated against the hormone-specific beta-subunit of hCG uas used with different labeled antigens to measure circulating hCG in patients having trophoblastic disease. When 125 I-hCGbeta served as the labeled antigen, a small number of patient sera failed to show parallelism with the second IS-hCG reference and erroneous estimates of hormone concentrations were obtained. Replacement of the 125I-hCGbeta with labeled hCG corrected the nonparallelism exhibited by these samples. Inhibition curves obtained with purified hCG and hCGbeta suggested that both the nonparallelism and its correction with the change in labeled antigen would be consistent with the possibility that this assay aberration may result from the presence of free hCGbeta in these sera.