The symptoms of growth hormone deficiency and growth hormone excess manifest themselves clinically in different ways, before and after the completion of longitudinal growth. Always, however, biochemical diagnostics is based on the measurement of circulating concentrations of growth hormone and insulin-like growth factor I (IGF-I). Immunoassays are practical, sensitive, and mostly specific methods for measuring either hormone. Still, there are serious discrepancies between the measured results of different assays. These discrepancies are mainly due to differences in the isoform specificity of assays, the use of different standard preparations, as well as the interference of binding proteins. The method-related differences in measured results make the general application of published diagnostic decision limits more difficult. At an interdisciplinary consensus conference, with the participation of the Growth Hormone Research Society, the IGF Society, and the International Federal of Clinical Chemistry and Laboratory Medicine (IFCC), the existing problems were analyzed and possible strategies were highlighted to improve the comparability of the measured results of different GH and IFG-I assays. Currently, however, the use of method-specific reference ranges obtained from well-characterized cohorts continues to be essential in clinical practice.