Endocrine Abstracts

Background: Fibroblast growth factor 23 (FGF23) is a key protein in bone homeostasis regulatingserumphosphateand calcitriol (1,25(OH)₂D₃) concentrations. Analytical performance and reference ranges of FGF23 assays differ by molecule assayed (intact or C-terminal FGF23) and manufacturer. We aim to establish reference interval values for both cFGF23 and iFGF23 in a paediatric population.

Methods: Written informed consent was taken from children aged 0 to 16 years with minimal past medical history attending a tertiary centre for a planned surgical procedure. Fasting serum and EDTA samples were collected and calcium intake and medical history recorded. Samples were analysed to measure plasma concentrations cFGF23 (Quidel, USA) and iFGF23 (DiaSorin, Italy), alkaline phosphatase, vitamin D, calcium, and phosphate. Unpaired t test was used to compare difference in analyte concentrations between males and females, children with different vitamin D and phosphate concentrations, and daily calcium intakes. Regression analysis was performed to assess change in analyte concentration with age.

Results: Eighty one females (mean age±S.D., 8.18±4.51) and 130 males (7.34±4.58) were included in the analysis. Plasma cFGF23 was significantly higher (P=0.025) in males (mean±S.D.,87.1±28.6RU/mL) than females (76.0±17.8RU/mL). iFGF23 was not significantly different (P=0.133) between males (37.3±12.4 pg/mL) and females (34.3±10.2 pg/mL). There is a non-significant downward trend in the concentration of cFGF23 and iFGF23 with increasing age in females (cFGF23:R2=0.136, F(1,41)=6.451,P=0.150; iFGF23:R2=0.044, F(1,56)=2.550,P=0.115), and males (cFGF23:R2=0.027, F(1,66)=1.801,P=0.184; iFGF23: R2=0.032, F (1,93)=3.110, P=0.081). Low total 25(OH)D (vitamin D) concentrations were significantly associated with decreased iFGF23 (P=0.020). Low phosphate concentration was significantly associated with decreased cFGF23 (P=0.015). Neither cFGF23 (P=)0.05 nor iFGF23 (P=0.61) values were associated with daily calcium intake.

Conclusion: We report values of cFGF23 and iFGF23 in a paediatric population with minimal past medical history. We have not yet reported reference intervals, numbers being likely to change as we analyse more samples. Of note, we observe sex differences in cFGF23 only, but no significant changes in cFGF23 and iFGF23 with age. The availability of cFGF23 and iFGF23 paediatric reference values will allow a better clinical use of these tests.