Growth hormone determination in children using an immunofunctional assay in comparison to conventional assays.

Conventional assays for determination of growth hormone (GH) in serum measure immunoreactive molecules of a blood sample. The immunofunctional assay (IFA), on the other hand, is able to determine biologically active molecules. In our study, we evaluated GH determination in children with IFA to compare these data with clinical reliable data of conventional assay systems, since there is only insufficient data concerning the clinical use of IFA in children. The comparison of GH determinations by IFA and two immunoradiometric assays showed different results for the same serum sample. Peak and trough concentration levels determined by these three different assays were always measured at the same time, but absolute GH concentration levels varied. Statistic analysis verified a linear regression of our data and allowed a conversion of data measured by IFA to predict values of the other assays used and vice versa. The traditional cut-off level for the diagnosis of GH deficiency of 10 ng/ml was based originally on results of polyclonal radioimmunoassays. This internationally applied cut-off level has been converted based on the regression analysis for prediction of this study and we found the 95% confidence interval on the mean measurements by IFA to be between 3.11 and 3.28 ng/ml.