Effects of growth hormone-binding proteins on serum growth hormone measurements.

GH-binding proteins (GH-BPs) in human blood theoretically may interfere with measurements of immunoreactive GH by forming complexes with GH and competing with antibody reagents for ligand. Indeed, results of serum GH obtained by immunoassays are known to differ markedly depending on the assay employed. To assess the potential role of circulating GH-BPs in this phenomenon, we systematically examined their effect on GH measurement in four RIAs and two immunoradiometric assays. In all except one RIA, the effect of the BPs on tracer binding to antibody was mildly inhibitory. In both immunoradiometric assays, BPs increased the nonspecific association of the tracer with the solid phase reagent. However, these effects were minor at the BP concentrations realistically encountered in practice. Furthermore, the impact of BPs on GH standard curves and final results was negligible because BP effects fall into the category of nonspecific or zero dose counts, which are subtracted during data reduction. We conclude that GH-BPs are only a minor disturbance in GH immunoassays, which is completely compensated for by conventional assay design. Disparities among results yielded by different assays are probably not due to BP interference.