Western ligand blot assay for human growth hormone-dependent insulin-like growth factor binding protein (IGFBP-3): the serum levels in patients with classical growth hormone deficiency

Failure of IGF-I and IGFBP-3 to diagnose growth hormone insufficiency

BACKGROUND

Growth hormone insufficiency (GHI) is diagnosed conventionally by short stature and slow growth, and is confirmed by diminished peak GH response to a provocation test. Insulin-like growth factor I (IGF-I) and IGF binding protein 3 (IGFBP-3) have previously been considered individually

OBJECTIVE

To test the hypothesis that the combined analysis of IGF-I and IGFBP-3 could act as a surrogate marker for the diagnosis of GHI.

DESIGN

Reference ranges for IGF-I and IGFBP-3 were calculated using 521 normal individuals. A retrospective analysis was performed on 318 children referred for investigation of short stature.

RESULTS

No significant difference was found between either the IGF-I or IGFBP-3 standard deviation scores (SDSs) in children with and without GHI. If the requirement were for both tests to be positive (< -2 SDS) for a diagnosis of GHI, then 99% of children without GHI would be correctly identified; however, the sensitivity of the test was only 15%.

CONCLUSIONS

Neither IGF-I nor IGFBP-3 alone is a marker for GHI. In addition, they cannot be used as an effective screening test in combination.

Prediction of the outcome of growth hormone provocative testing in short children by measurement of serum levels of insulin-like growth factor I and insulin-like growth factor binding protein 3

Serum levels of insulin-like growth factor I (IGF-I) and insulin-like growth factor binding protein 3 (IGFBP-3) reflect the secretion of endogenous growth hormone (GH) in healthy children and exhibit little diurnal variation, which makes them potential candidates for screening of GH deficiency (GHD). We evaluated serum IGF-I and IGFBP-3 levels in relation to the outcome of GH provocative testing in 203 children and adolescents (111 boys and 92 girls) in whom GHD was suspected. A total of 1030 children served as control subjects. In children less than 10 years of age, IGF-I levels were below the cutoff limit in 8 of 15 children with GHD (sensitivity 53.3%) and above the cutoff limit in 47 of 48 children with a normal GH response (specificity 97.9%). Similarly, IGFBP-3 levels were below the cutoff limit in 9 of 15 children with GHD (sensitivity 60%) and above the cutoff limit in 47 of 48 children with a normal GH response (specificity 97.9%). Consequently the predictive value of a positive test result in prepubertal children was 88.8% for IGF-I and 90% for IGFBP-3. In children and adolescents between 10 and 20 years of age, IGF-I levels were below the cutoff limit in 34 of 46 children with GHD (sensitivity 73.9%) and above the cutoff limit in 63 of 94 children with normal GH response (specificity 67%). IGFBP-3 levels were below the cutoff limit in 26 of 46 children with GHD (sensitivity 56.5%) and above the cutoff limit in 74 of 94 children with a normal GH response (specificity 78.7%). Accordingly the positive predictive value and in 10- to 20-year-old children was 52.3% for IGF-I and 56.5% for IGFBP-3. Combination use of IGF-I and IGFBP-3 gave additional diagnostic information. We conclude that a subnormal IGF-I level, and especially a subnormal IGFBP-3 level, are highly predictive of a subnormal GH response to a provocative test in prepubertal children in whom GHD is suspected. On the other hand, a normal IGF-I or IGFBP-3 level does not exclude GHD. The predictive value of IGF-I and IGFBP-3 in pubertal children is diminished in comparison with that in prepubertal children. We believe that IGF-I and IGFBP-3 are valuable tools in the evaluation of childhood GHD.

Measurement of free insulin-like growth factor-I using immunoradiometric assay

The free Insulin-like growth factor-I (IGF-I) in plasma from normal adults was directly measured with a newly developed highly sensitive immunoradiometric assay (IRMA) for IGF-I. The capture antibody did not crossreact with IGF-I associated binding proteins which exist in plasma, and the assay was designed not to shift the equilibrium of the IGF-I and binding proteins. Total IGF-I concentration was measured using this assay with preliminary acid-ethanol extraction. Approximately 1 percent of total IGF-I existed in the free form. Gel filtration of plasma was also used to separate the free IGF-I from its bound form. The free/total ratio of IGF-I as determined by gel filtration was similar to that determined directly by IRMA with and without acid-ethanol extraction.