Laboratory measurement of growth hormone.

Growth hormone (GH) measurements are complicated by the heterogeneous nature of GH, as well as by the presence of the GH binding protein in plasma. Several isoforms of GH exist, and specific assays for each are currently either unavailable, impractical, or not clinically indicated. Bioassays include the in vivo assays based on rat weight gain, tibial line widening, or IGF-I generation. In vitro bioassays, based on the proliferation of cell lines expressing the prolactin receptor or GH receptor, are sensitive but prone to nonspecific interference by factors present in serum. Immunoassays (RIA, IRMA, ELISA, and immunofunctional assay design) are widely used in the clinical laboratory because of speed, sensitivity, and convenience. Discrepancies among results rendered by different immunoassays have become more apparent as monoclonal assays have superseded polyclonal assays, presumably because different antibodies recognize different epitopes among the heterogeneous mixture of GH isoforms in serum. Some assays, especially those with short, nonequilibrium incubation times are vulnerable to interference by the GH binding protein present in serum. Recommendations are given for strategies designed to minimize disparity of results obtained by different GH immunoassays applied to serum. Urinary GH measurements, while technically feasible, are of limited clinical utility because of biological variation in urinary GH excretion.