A study of IGF-1 serum concentrations and kinetics of growth hormone assays in stimulation tests used for diagnosing insufficient somatotrope secretion
Introduction: The diagnosis of insufficient somatotrope secretion (growth hormone deficiency or GHD) is important in children.
In France, a child is considered to have a deficit if no serum GH concentrations greater than 20 mIU/l are recorded in 2 different stimulation tests. One of the tests must be coupled, i.e. combining 2 pharmacological agents.
However, this 20 mIU/l threshold does not take account of the type of test and large response variances that can be reported, depending on the product used. Nor does it factor in the patients weight, age and pubertal development.
The goal of this study was to evaluate the interpretation of these tests vis-à-vis the legal threshold of 20 mIU/l depending on the product administered, and to compare it to IGF-1 results obtained, according to the standard reference values given by the supplier according to age.
Materials and methods: Chemiluminescent assays of hGH were performed in our laboratory in 2007 using Beckman Coulters DXI instrument and somatomedin C assays were conducted using the Immulite 2000 from DPC (Siemens). HGH assays were calibrated to the international standard IS 98/574, as recommended.
Results: Of the 430 dynamic tests and regardless of the product(s) used, only 50% showed a positive response, with a secretion peak above 20 mIU/l. This percentage varied according to the product used:
ornithine (51.3%), insulin (22.8%), glucagon (42.8%), L-Dopa (50%)
clonidinebetaxolol (28.6%), glucagonbetaxolol (64.8%), glucagonpropanolol (69%), kerloneglucagon (65.5%)
In light of these results and because the highest GH concentrations are observed at very different times from one patient to the next, it is important to comply with the different protocol times to avoid overlooking an unrecorded peak.
Despite the relationship between GH and IGF-1, the percentage of low IGF-1 concentrations in patients with a positive response to dynamic tests (15% of the 73 tests with ornithine) does not differ considerably from that observed in those presenting a response <20 mIU/l (19.7% of the 66 tests with ornithine). Therefore, IGF-1 assays cannot be used as a substitute for dynamic tests in diagnosing a GHD.
Keywords: hGH, IGF1, dynamic tests, GHD