Influence of growth hormone binding protein on growth hormone estimation in different immunoassays

Growth hormone (GH) quantitation in biological fluids varies depending on the assays employed, and factors which may interfere in the assays include the high affinity GH-binding protein (GHBP). To evaluate this potential effect on GH estimates, we studied the influence of adding increasing amounts of high affinity glycosylated GHBP to normal, acromegalic and GH-deficient sera, which were then processed in four different immunoassays. Two commercial immunometric assays, Delfia and Nichols (assays 1 and 2), and two RIAs, one using a polyethylene glycol (PEG) precipitation (assay 3) and one using wick-chromatography (assay 4) for separation of free and bound 125I-GH, were employed. In the Delfia assays, GH estimates of 11 sera decreased (p < 0.05) to 87.2 +/- 2.6%, 73.0 +/- 2.7% and 60.1 +/- 2.5% (mean +/- SEM) of basal GH estimates with the addition of GHBP in concentrations of 0.54, 2.14 and 6.42 nmol/l, respectively. In the Nichols assay, GH estimates were not significantly reduced (93.4 +/- 2.6%, 83.8 +/- 4.5% and 83.9 +/- 3.9%) with the applied GHBP concentrations. In assay 3 (RIA), the addition of GHBP increased GH estimates to 122 +/- 10.0% and 167 +/- 19.1% (both p < 0.05) with the addition of GHBP in concentrations of 2.14 and 6.42 nmol/l, respectively, whereas an increase in GHBP concentration of 0.54 nmol/l did not change the estimates from basal levels (99.0 +/- 4.8%, p > 0.05). In assay 4 (RIA), the addition of GHBP induced decreased GH estimates. With this varying influence of GHBP on GH estimates, binding protein interference should be taken into consideration when comparing GH estimates obtained with many currently utilized GH immunoassays. The present results demonstrate that GHBP levels within physiological range may interfere with the results of GH assays, giving either spuriously high or low values depending on the GH assay methodology.