Reference Values for IGF-I Serum Concentrations: Comparison of Six Immunoassays

CONTEXT: Measurement of IGF-I is essential for diagnosis and management of patients with disorders affecting the somatotropic axis. However, even when IGF-I kit manufacturers follow recent consensus guidelines, different kits can give very different results for a given sample. OBJECTIVES: We sought to establish normative data for six IGF-I assay kits based on a large random sample of the French general adult population. SUBJECTS AND METHODS: In a cross-sectional multicenter cohort study, we measured IGF-I in 911 healthy adults (18–90 years) with six immunoassays (iSYS, LIAISON XL, IMMULITE, IGFI RIACT, Mediagnost ELISA, and Mediagnost RIA). Pairwise concordance between assays was assessed with Bland-Altman plots for both IGF-1 raw data and standard deviation scores (SDS), as well as with the percentage of observed agreement and the weighted Kappa coefficient for categorized IGF-I SDS. RESULTS: Normative data included the range of values (2.5–97.5 percentiles) given by the six IGF-I assays according to age group and sex. A formula for SDS calculation is provided. Although the lower limits of the reference intervals of the six assays were similar, the upper limits varied markedly. Pairwise concordances were moderate to good (0.38–0.70). CONCLUSION: Despite being obtained in the same healthy population, the reference intervals of the six commercial IGF-1 assay kits showed noteworthy differences. Agreement between methods was moderate to good.