Characteristics and reference ranges of Insulin-Like Growth Factor-I measured with a commercially available immunoassay in 724 healthy adult Caucasians

Background and Objective: Measurements of Insulin-Like Growth Factor-I (IGF-I) play a pivotal role in the evaluation of the growth hormone-IGF-I axis. Due to assay variation IGF-I reference ranges are assay specific. We provide serum IGF-I reference ranges for adult men and women obtained by a commercially available assay. Method: IGF-I was measured by an enzyme-linked immunosorbent assay (R&D Systems). Assay precision was evaluated in low, medium and high IGF-I pools and in single samples from outpatients. The reference ranges were obtained in 724 healthy Caucasians, mean age 48 years (range 19-91). Results: IGF-I was measured higher in serum compared to plasma samples (p < 0.001, R-2 = 0.96). To convert EDTA plasma values to serum values a factor of 1.22 was calculated. The intraassay coefficients of variation (CV) for the low: 56.4 +/- 2.7 ng/mL, medium: 179.7 +/- 5.9 ng/mL and high pool: 445.61 +/- 3.2 ng/mL (mean 1SD) were 5, 3 and 3%. Interassays CVs for the low, medium and high pool varied between 7-10, 5-7, and 6-9%. Reproducibility between 4 different lots showed a intraclass CV of 0.99 (95% CI 0.98-0.96). Logarithmically transformed IGF-I levels were linearly associated with age with a 13% reduction in IGF-I per decade in females and 11% in males. Conclusion: We have provided IGF-I reference ranges obtained by an assay that showed variations and reproducibility that was considered of sufficient high quality for clinical and scientific use.