Two-site immunofluorometric assay for pregnancy-specific beta 1-glycoprotein (SP1)

We describe an immunoassay for human pregnancy-specific beta 1-glycoprotein by use of fluorescein-labeled antibodies to this protein. The technique is generally similar to that of a "two-site" immunoradiometric assay but has the advantage of stability of tracer and avoidance of radiation hazard. However, the procedure requires relatively large quantities of antibody. Furthermore, the within-assay coefficient of variation (11.8%) is greater than that for radioimmunoassay (6.5%). This results from imprecision of the end-point detection, and some inherent difficulties in optimizing a "two-site" immunometric system. Concentrations of endogenous material as measured by the fluorometric assay are 1.9-fold higher than those obtained with radioimmunoassay, probably because the former detects multiple forms of this heterogeneous material.