Isotopic and nonisotopic assays for measuring somatotropin compared: re-evaluation of cutoff value in provocative tests

Measurement of human growth hormone (hGH; somatotropin) concentrations in serum after provocative tests is crucial for diagnosing deficiencies in production of this hormone. Serum hGH can be measured by various immunoassays, isotopic and nonisotopic, with monoclonal or polyclonal antibodies: a cutoff value of 10 micrograms/L after provocative testing is usually used to distinguish normal from hGH-deficient children. Previous studies demonstrated discrepancies in hGH measurement by different radioisotopic immunoassays. Here we evaluated the responses of six different commercial assays, radioisotopic and nonisotopic, with monoclonal or polyclonal antibodies in a series of 16 provocative tests (stimulation with clonidine) in short children. A wide range of discrepant values was obtained with the different kits. A cutoff of 10 micrograms/L produced discordance of diagnosis among assays for two children, whereas complete agreement was reached for a cutoff value of 7 micrograms/L. Parallelism tests performed with hGH international standard, pure recombinant hGH, and a serum with high hGH content suggest that heterogeneity of the antibodies used by the manufacturers, even among monoclonal antibodies, is the main source of discordant results. Cutoff values and reference values must be established separately for each method proposed for routine use.