Concordance between direct and indirect measurements of free 25-OH vitamin D

• Published in: Clinica chimica acta Vol. 475; pp. 169 - 171
• Main Authors: Peris, Pilar, Filella, Xavier, Monegal, Ana, Guañabens, Nuria, Foj, Laura, Bonet, María, Boquet, Dolors, Casado, Enrique, Cerdá, Dacia, Erra, Alba, Gómez-Vaquero, Carmen, Martínez, Silvia, Montalá, Nuria, Pittarch, Concepción, Kanterewicz, Eduardo, Sala, Miquel, Suris, Xavier, Carrasco, Josep L.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2017
• Subjects: Adult; Bioavailable vitamin D; Calcifediol - blood; Enzyme-Linked Immunosorbent Assay - standards; Female; Free vitamin D; Humans; Middle Aged; Regression Analysis; Reproducibility of Results; Vitamin D binding protein; Vitamin D Deficiency - blood; Vitamin D Deficiency - diagnosis; Vitamin D-Binding Protein - blood; Bioavailable vitamin D; Vitamin D binding protein; Free vitamin D
• ISSN: 0009-8981; 1873-3492; 1873-3492
• DOI: 10.1016/j.cca.2017.10.016

At present, data comparing the quantification methods for measurement of free vitamin D (direct assay [direct 25-OHDF] and estimated by calculation [calculated 25-OHDF]), are scarce. The aim of this study was to analyse the concordance between these two methods of 25-OHDF analysis (direct vs. calculated). Serum values of total 25-OHD (25-OHDT), vitamin D binding protein (DBP) (by R&D Systems ELISA), calculated 25-OHDF (by DBP, albumin and 25-OHDT) and direct 25-OHDF (by DIAsource ELISA) were analysed in 173 healthy women (aged 35–45years). Assessment of concordance was evaluated by the Bland-Altman plot and the total deviation index (TDI). The mean values of calculated and direct 25-OHDF in these subjects were 5.27±2.5 and 3.83±1.01pg/mL, respectively. We found significantly lower values of 25-OHDF on comparing subjects with and without vitamin D deficiency, independently of the method used. The total deviation index evaluated by the Bland-Altman plot showed low concordance for both measurements. Only low 25-OHDF levels were concordant. This study shows that the concordance between these two methods of 25-OHDF analysis is low and has a concentration dependent bias. Further studies are necessary to clarify the reference values and the indications for 25-OHDF measurement. •Measurement of free 25-OHD by a direct assay seems to be a useful tool for evaluating this vitamin D fraction.•The concordance between direct and indirect measurements of free 25-OHD is low when using a monoclonal antibody based assay.•It is necessary to clarify the reference values and indications for free 25-OHD measurement.