Pitfalls of insulin-like growth factor I assays.

Background: Immunoassays that measure circulating concentrations of insulin-like growth factor I (IGF-I) are increasingly used to diagnose and monitor growth hormone (GH)-related diseases. Specifically, IGF-I measured by immunoassay is used to diagnose GH deficiency and acromegaly, and to monitor treatment efficacy in patients with acromegaly, particularly those treated with the GH receptor antagonist pegvisomant, as measurement of circulating GH is no longer suitable for monitoring disease activity. While techniques for measuring IGF-I have evolved over the decades, immunoassays are still the primary tool used in routine laboratories. Immunoassays depend on the interaction between antibodies and the analyte, and all factors that modify the accessibility of the epitopes recognized by the antibodies can influence results. With IGF-I assays, interference from binding proteins is an important variable affecting assay results.

Conclusions: It is generally accepted that assay- and age-specific reference ranges are mandatory for meaningful interpretation of IGF-I concentrations. High-quality, method-specific reference ranges and a high degree of methodological consistency in the assay are essential for reliable comparison of results across studies and for long-term monitoring of individual patients.