IMMUNOFUNCTIONAL ASSAY OF HUMAN GROWTH-HORMONE (HGH) IN SERUM - A POSSIBLE CONSENSUS FOR QUANTITATIVE HGH MEASUREMENT

Confirmation of the diagnosis of GH deficiency in adults and children involves provocative testing for human (h) GH. Different commercially available immunoassays yield largely discrepant results in the measure ment of GH levels in human serum. These discrepancies result in doubtf ul relevance of cut-off levels proposed for GH provocative testing. We have developed an immunofunctional assay method that allows quantitat ion of only those GK forms in circulation that possess both binding si tes of the hormone for its receptor and thus can initiate a biological signal in target cells. An anti-hGH monoclonal antibody recognizing b inding site 2 of hGH is immobilized and used to capture hGH from the s erum sample. Biotin-labeled recombinant GH-binding protein in a second incubation step forms a complex with those hGH molecular isoforms tha t have both binding sites for the receptor. The signal is detected aft er a short third incubation step with labeled streptavidin. The assay is sensitive (detection range, 0.1-100 mu g/L) and has average inter- and intraassay precisions of 10.3% and 7.3%, respectively. Endogenous GH-binding protein does not interfere with the hGH result; placental l actogen shows no detectable cross-reaction in this immunofunctional as say. The degree of immunofunctionally active hGH forms in serum sample s, calculated by comparison of immunofunctional assay and RIA results, varied between 52-93%. We propose this immunofunctional assay for GH measurement as a new reference method for hGH quantitation in serum. T he immunofunctional assay translates only hGH forms into an assay sign al that are capable of dimerizing GH receptors and, thus, of initiatin g a biological effect in target cells.