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Validation of Functional Insulin-Like Growth Factor Binding Protein-3 Measurement by a Ligand Immunoassay by M. Langkamp, K. Weber, M. Kirschner, L. Pridzun, M.B. Ranke

Background: Proteolysis of Insulin-like Growth Factor Binding Protein (IGFBP) – 3 is a well known mechanism regulating IGF-I bioavailability and IGF-independent actions of IGFBP-3 fragments. Measurement of functional IGFBP-3 can be of use in diagnostics of growth failure or renal impairment. We herein characterize the properties of a commercially available immunoassay for the measurement of functional (IGF-I binding) IGFBP-3.
Method: Fragmentation of IGFBP-3 is analyzed by gel filtration, SDS-PAGE, and western ligand and immunoblotting and compared with subsequent measurement of total and functional IGFBP-3 by ELISA/IFA. Furthermore, assay characteristics such as reproducibility, linearity, and sensitivity are surveyed.
Results: Functional IGFBP-3 was reproducibly measured (6.8/5.6 % Inter-/Intra assay variance). A broad range of linearity (1:50 – 1:300) and a high sensitivity (0.18 µg/L) allowed reliable measurement of IGF-binding IGFBP- 3. Analysis of IGFBP-3 fragments reveals that the assay described only detects intact IGFBP-3. Analysis of 189 serum samples from healthy blood donors showed that on average 84 % and 69 % of total IGFBP-3 was functional in men and women, respectively (p < 0.01).
Conclusions: Functional IGFBP-3 can be measured reliably by the assay system used. Thus, this assay system is suited for the investigation of the diagnostic value of functional IGFBP-3 in human body fluids.

DOI: Clin. Lab. 2010;56:535-542