Laboratory investigations in the diagnosis and follow-up of GH-related disorders

Abstract

In addition to auxiological, clinical and metabolic features measurements of growth hormone (GH) and insulin-like growth factor I (IGF-I) complement our tools in diagnosis and follow-up of GH-related disorders. While comparably robust during the pre-analytical phase, measurement and interpretation of concentrations of both hormones can be challenging due to analytical issues and biological confounders. Assay methods differ in terms of antibody specificity, interference from binding proteins, reference preparations and sensitivity. GH assays have different specificity towards different GH-isoforms (e.g. 20 kDa GH, placental GH) and interference from the GH antagonist Pegvisomant. The efficacy to prevent binding protein interference is most important in IGF-I assays. Methodological differences between assays require that reference intervals and diagnostic cut-offs are assay-specific. Among biological variables, pubertal development and age are most relevant for IGF-I, making detailed reference intervals mandatory for interpretation. GH has pulsatile secretion and short half-life. Its concentration is modified by acute factors such as stress, exercise and sleep, but also by intake of oral estrogens and anthropometric factors (e.g. BMI). Other GH dependent biomarkers such as free IGF-I, IGF binding protein 3 (IGFBP 3) and acid labile subunit (ALS) have been proposed. Their concentrations largely mirror the information obtained through measurement of IGF-I, but their measurement can be helpful in particular situations. In this review, we describe the evolution of analytical methods to measure biomarkers of GH action, the impact of the methodological changes on laboratory results and the need to include biological variables in their interpretation. Arch Endocrinol Metab. 2019;63(6):618-29.

Figure 1. Measurement of GH in the same two samples A and B by different laboratories (n=208). Reported concentrations vary by more than 100% ( 1 ). The same results split by manufacturer reveal systematic differences between the respective assay methods. Three of the automated assays are shown as an example ( 2 ). Results taken from the External Quality Assessment Scheme 4/2017 organized by Reference Institute for Bioanalytics (RfB, Bonn, Germany), one of the two German proficiency testing organizations. More results can be accessed at http://www.rfb.bio

Figure 2. IGF-I immunoassay scheme. The ternary complex ( 1 ) is dissociated by acid extraction ( 2 ), but re-aggregation of the components during incubation can impair antibody binding to IGF-I ( 3 ). Addition of excess IGF-II blocks binding protein interference ( 4 ).

IGF-I: insulin-like growth factor I; IGF-II: insulin-like growth factor II; IGFBP-3: insulin-like growth factor binding protein-3; ALS: acid labile subunit; BP: binding protein.