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ABSTRACT
In a retrospective analysis, we have compared the response of serum GH concentration to insulin-induced hypoglycaemia in 148 short prepubertal children (114 males, 34 females) aged between 3·9 and 11·9 years with the growth rate of the individual to determine 'cut-off' values for the diagnosis of GH insufficiency.
Sixty-three children grew with a height velocity standard deviation score (SDS) greater than −0·8 (group 1), which represents the growth velocity of children progressing along or closely parallel to the third height centile. Eighty-five children had a height velocity SDS of less than −0·8 (group 2). Median peak serum GH concentration responses to insulin-induced hypoglycaemia were 19·9 mU/l (range 1·5–54·4) in group 1 and 9·9 mU/l (range 0·7–46·2) in group 2 (Mann–Whitney; P < 0·001).
Using growth rate as the determinant of normality, the efficiency, sensitivity and specificity of the insulin-induced hypoglycaemia test were calculated using different serum GH concentration cut-off values to diagnose GH insufficiency. In our (Hybritech) assay, a cut-off value of 13·5 mU/l provided optimal performance in terms of efficiency (66%), sensitivity (64%) and specificity (70%).
The response of serum GH concentration to insulin-induced hypoglycaemia in short children growing at different growth rates was continuous. Each laboratory measuring serum GH concentrations needs to construct its own 'normal' cut-off value.
Journal of Endocrinology (1992) 133, 447–450
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ABSTRACT
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
