Research reportComparative assays in three laboratories of serum specimens containing pregnancy-specific β1 glycoprotein with different proportions of the α variant

Following several reports that the α component of pregnancy-specific β1 glycoprotein (SP1) gives dose-response curves that are flatter than those of the β component when assayed by radioimmunoassay of the competitive type, a collaborative study was carried out by 3 laboratories to examine whether radial immunodiffusion or rocket immunoelectrophoresis give a similar result. Five sera from pregnant women at the 34–39th week of gestation with β : α ratios of 0.8–4.7 were assayed by both the latter methods against either the IARC 78 610 reference preparation or the SP1 Behring standard. When preparations were assayed by rocket immunoelectrophoresis (two laboratories), the dose-response curves were parallel. When preparations were assayed by radial immunodiffusion (two laboratories), parallelism was observed (one laboratory) after the addition of polyethylene glycol, the presence of which is necessary to visualize immune precipitates in which α is involved. Discrepancies observed when methodologies such as competitive radioimmunoassay are used, in which there are limited amounts of antibodies, are explained, at least partially, by the fact that the β component has a greater relative affinity for the antibody as higher concentrations of β : α mixtures are assayed. However, when antibody is present in excess, as in radial immunodiffusion and rocket immunoelectrophoresis, no such difference in affinity seems to occur.