Normal Values of Circulating Insulin-Like Growth Factor-I Bioactivity in the Healthy Population: Comparison with Five Widely Used IGF-I Immunoassays

Background: IGF-I immunoassays are primarily used to estimate IGF-I bioactivity. Recently an IGF-I-specific kinase receptor activation assay (KIRA) has been developed as an alternative method. However, no normative values have been established for the IGF-I KIRA. Objective: The objective of the study was to establish normative values for the IGF-I KIRA in healthy adults. Design: This was a cross-sectional study in healthy nonfasting blood donors. Study Participants: Participants included 426 healthy individuals (310 males, 116 females; age range 18–79 yr). Main Outcome Measures: IGF-I bioactivity determined by the KIRA was measured. Results were compared with total IGF-I, measured by five different IGF-I immunoassays. Results: Mean (± sd) IGF-I bioactivity was 423 (± 131) pmol/liter and decreased with age (β = −3.4 pmol/liter·yr, P < 0.001). In subjects younger than 55 yr, mean IGF-I bioactivity was significantly higher in women than men. Above this age this relationship was inverse, suggesting a drop in IGF-I bioactivity after menopause. This drop was not reflected in total IGF-I levels. IGF-I bioactivity was significantly related to total IGF-I (rs varied between 0.46 and 0.52; P < 0.001). Conclusions: We established age-specific normative values for the IGF-I KIRA. We observed a significant drop in IGF-I bioactivity in women between 50 and 60 yr, which was not perceived by IGF-I immunoassays. The IGF-I KIRA, when compared with IGF-I immunoassays, theoretically has the advantage that it measures net effects of IGF-binding proteins on IGF-I receptor activation. However, it has to be proven whether information obtained by the IGF-I KIRA is clinically more relevant than measurements obtained by IGF-I immunoassays.