Reference values for IGF-I serum concentration in an adult population: use of the VARIETE cohort for two new immunoassays.

Context Measurement of IGF-I is important in the management of patients with growth hormone disorders. Here we aim to establish normative data for two new IGF-I assay kits based on a large random sample of the French general adult population. Subjects and methods We measured IGF-I in 911 healthy adults (18-90 years) with two immunoassays (ROCHE Elecsys® and IMMULITE-2000 calibrated against the new IS 02/2547). We compared the data with those of the six immunoassays (iSYS, LIAISON XL, IMMULITE-2000 calibrated against the first IS 87/518, IGFI RIACT, Mediagnost ELISA, and Mediagnost RIA) that we reported previously. The pairwise concordance among the eight assays was assessed with Bland-Altman plots for both the IGF-1 raw data and the standard deviation scores (SDS), as well as with the percentage of observed agreement and the weighted Kappa coefficient for categorizing IGF-I SDS (ClinicalTrials.gov Identifier: NCT01831648). Results The normative data included the range of values (2.5-97.5 percentiles) given by the two new IGF-I assays according to age group and sex. A formula for the SDS calculation is provided. For the previous six assays, the lower limits of the reference intervals of the two new assays were similar, but the upper limits varied markedly. The pairwise concordances were only moderate (kappa 0.57). Conclusions Data obtained for these two new IGF-I immunoassays confirm that despite being obtained in the same large healthy population, the reference intervals of the eight commercial IGF-1 assay kits showed noteworthy differences. The agreement among the various methods was moderate to good.