Objective: Circulating GH-binding protein (GHBP) may interfere with GH measurements in immunoassays by competing with the antibodies for ligands. The concentrations of circulating GHBP in humans are closely related to the amount of body fat, and subject to large interindividual differences.
Subjects and methods: To assess the influence of GHBP in a widely used commercial immunometric GH kit (DELFIA, Wallac, Finland) we systematically tested the effects of varying GHBP concentrations and assay incubation times on GH estimates over a broad range of GH concentrations. We subsequently investigated the impact of 24-h vs. 2-h incubation on estimates of GH pharmacokinetics obtained from two-step primed-constant infusions of GH in a group of 26 healthy nonobese men [mean age 37.3 years (range 22-55); body mass index (BMI) = 24.6 +/- 0.4 kg/m2].
Results: GHBP at physiological concentrations of 0.2, 0.5, 1.0, 2.0 nm reduced the GH estimates by as much as 40% at low GH concentrations. By increasing the incubation time from the recommended 2 h to 24 h the interference from GHBP was almost completely eliminated. The increase in measured GH using 24-h vs. 2-h incubation showed a strong positive correlation to the subjects' GHBP levels (r = 0.66, P < 0.001). Consequently, the estimates of metabolic clearance rate (MCR) of GH at constant infusion rates of 1.5 micro g/kg/h and 3.0 micro g/kg/h were significantly reduced when using 24 h as opposed to 2 h incubation, and the changes were negatively correlated to the GHBP levels (r = -0.62, P < 0.001 and r =-0.54, P < 0.01, respectively). Furthermore, by reducing the interference of GHBP through 24-h incubation, the previously observed positive correlations between MCR and not only the subjects' GHBP levels but also with total body fat were reduced, while the positive correlation between baseline insulin concentrations and GH clearance was strengthened.
Conclusions: We conclude that differences in GHBP concentrations significantly influence GH measurements in this commercial immunoassay, and that interindividual differences in GHBP concentrations may interfere with the results in studies involving between-subject comparisons of GH concentrations and pharmacokinetics. We believe the extended incubation time allows for better 'extraction' of GH bound to serum GHBP, and that this effect should be investigated, and if relevant, be exploited in other GH immunoassays.