A simple, rapid immunometric assay for determination of functional and growth hormone-occupied growth hormone-binding protein in human serum

We present a sensitive time-resolved fluorometric immunofunctional assay (TR-FIA) for direct quantitation of functional growth hormone-binding protein (GHBP), using an immunoassay kit for growth hormone (GH-DELFIA). In addition to the immobilized GH antibody, one monoclonal antibody against GHBP was used. This anti-GHBP was labelled with the chelate of europium. The assay was performed in one step. The detection limit for GHBP was 0.044 nmol L-1 (NBS + 3 SD). The calibration curve was linear in the interval 0.11-8.03 nmol L-1. Average intra-assay coefficient of variation (CV) was 3.44%. Average interassay CV at GHBP concentrations 0.563 nmol L-1 and 1.40 nmol L-1 were 12% and 6.3% respectively. Analytical recovery in serum ranged from 76% to 127% with a mean of 101 +/- 3.6%. Serum GHBP in 102 normal subjects ranged from 0.513 to 3.772 nmol L-1 and was positively related to body mass index (P < 0.001). In growth hormone-deficient sera GHBP was higher than in control subjects (1.751 +/- 0.179 nmol L-1 and 1.257 +/- 0.140 nmol L-1 respectively, P < 0.001). Acromegalic patients had lower levels of GHBP than controls (0.946 +/- 0.251 and 1.234 +/- 0.144 nmol L-1 respectively, P = 0.005). This assay also allowed detection of GH-complexed GHBP in serum. These results were in agreement with theoretical values calculated from the measured GH and the functional GHBP concentrations. Results were compared with data obtained by a recently reported, validated ligand immunofunctional assay (LIFA), which is fundamentally different. There was a significant linear relationship between the results from the two assays (r = 0.89, P = 0.001). The slope of the regression line was 0.65. In conclusion, this new convenient GHBP TR-FIA provides a sensitive and precise method for detecting total GHBP as well as complexed GHBP in human serum, and allows easy processing of large numbers of samples.