Comparative assays in three laboratories of serum specimens containing pregnancy-specific beta 1 glycoprotein with different proportions of the alpha variant.

Following several reports that the alpha component of pregnancy-specific beta 1 glycoprotein (SP1) gives dose-response curves that are flatter than those of the beta 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 similar result. Five sera from pregnant women at the 34-39th week of gestation with beta : alpha ratios of 0.8-4.7 were assayed by both the latter methods against either the IARC 78 610 reference preparation of 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 alpha 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, but the fact that the beta component has a greater relative affinity for the antibody as higher concentrations of beta : alpha 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.