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 measurement of GH levels in human serum. These
discrepancies result in doubtful relevance of cut-off levels
proposed for GH provocative testing. We have developed an
immunofunctional assay method that allows quantitation of only
those GH forms in circulation that possess both binding sites of
the hormone for its receptor and thus can initiate a biological
signal in target cells. An anti-hGH monoclonal antibody recognizing
binding site 2 of hGH is immobilized and used to capture hGH from
the serum sample. Biotin-labeled recombinant GH-binding protein in
a second incubation step forms a complex with those hGH molecular
isoforms that have both binding sites for the receptor. The signal
is detected after a short third incubation step with labeled
streptavidin. The assay is sensitive (detection range, 0.1-100
micrograms/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 lactogen slows no
detectable cross-reaction in this immunofunctional assay. The
degree of immunofunctionally active hGH forms in serum samples,
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. The immunofunction assay translates only hGH forms into an
assay signal that are capable of dimerizing GH receptors and, thus,
of initiating a biological effect in target cells.