ESPE Abstracts (2018) 89 RFC2.3

Measured Free 25-Hydroxyvitamin D in Healthy Children and Relationship to Total 25-Hydroxyvitamin D, Calculated Free 25-Hydroxyvitamin D and Vitamin D Binding Protein

Laura Bosch i Araa, Maria Lopez-Molinab, Cecilia Santillanc, Marta Murilloa, Aina Vallsa & Joan Bela

aPaediatrics Department, Germans Trias i Pujol University Hospital, Badalon, Badalona, Spain; bClinical Biochemistry Department, Germans Trias i Pujol University Hospital, Badalona, Spain; cGermans Trias i Pujol University Hospital, Badalona, Spain.

Introduction: Vitamin D deficiency in children is still a global health problem. Measuring free 25-hydroxyvitamin D concentrations could provide a better estimate of the vitamin D status than total 25-hydroxyvitamin D (25(OH)D) levels.

Objective: To assess the relationship between measured free vitamin D (m-f25(OH)D) and calculated free 25(OH)D (c-f25(OH)D), total 25(OH)D, intact parathyroid hormone (iPTH) and other markers of phosphocalcic metabolism. Establish serum m-f25(OH)D concentrations corresponding to a total 25(OH)D>20 ng/ml which is accepted as vitamin D-sufficiency status in children.

Material and Methods: Prospective cohort study setted between January and February 2017 in healthy children of a Mediterranean population The measurements of m-f25(OH)D and vitamin D binding protein (VDBP) were made by ELISA. Free 25(OH)D was calculated using the formula described by Bikle.

Results: m-f25(OH)D directly correlated with total 25(OH)D (r:0.804, P<0.001), serum calcium (r:0.26, P:0.035), and c-f25(OH)D (r:0.553, P:0.016); and inversely with iPTH (r:−0.374, P:0.002), alkaline phosphatase (r:−0.28, P:0.026), and age (r:−0.289, P:0.018). Total 25(OH)D correlated with the same parameters than m-f25(OH)D except for serum calcium. Whereas, c-f25(OH)D correlated only with total 25(OH)D and VDBP, both included in the calculation formula. Multiple regression analysis showed that m-f25(OH)D variations were independently explained by calcium (β:0.156, P:0.026) and total 25(OH)D (β:0.043, P<0.001). The optimal m-f25(OH)D cut-off for discriminating between insufficient and sufficient total 25(OH)D was ≥3.9 pg/ml (Area Under Curve (AUC): 0.897 (95% confidence interval (CI): (0.798–0.958); P<0.001; sensitivity:72.7% (95%CI: 49.8–89.3); specificity: 95.4% (95%CI: 84.5–99.4))

Conclusions: Directly measured free vitamin D correlated better with markers of phosphocalcic metabolism than total 25(OH)D and c-f25(OH)D in a population of healthy children.

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