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We have developed methods for measuring the concentrations of free GH in plasma using a polyethylene glycol (PEG) separation procedure to remove antibody-bound GH within 1 h of collection. Total GH concentrations were obtained by acidification of the GH–antibody complex to release the GH followed by PEG precipitation of the antibody. The plasma GH assay had a within-assay coefficient of variation (C.V.) of 6·8% at 4·6 mU/l and a between-assay C.V. of 9·2% at 4·0 mU/l. The PEG-modified assay had a within-assay C.V. of 4·3% at 6·3 mU/l and a between-assay C.V. of 10·9% at 5·3 mU/l. Both assays had a sensitivity of 1·3 mU/l. There was good correlation between plasma and free GH concentrations in 24-h profiles in two tall children (r = 0·98; P < 0·001) and between total and free GH in the same profiles (r = 0·97; P < 0·001).

GH antibodies were measured using a highly sensitive radioimmunoassay. In children who did not develop GH antibodies there was no difference between total, plasma and free GH concentrations. In contrast, in those who developed GH antibodies both total and plasma GH concentrations were markedly increased compared with free GH concentrations. The presence of GH antibodies did not affect the growth, plasma insulin-like growth factor-I concentrations or fasting serum insulin concentration responses to 1 year of therapy with biosynthetic human GH.

Journal of Endocrinology (1989) 121, 193–199