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. 2017 Dec:475:169-171.
doi: 10.1016/j.cca.2017.10.016. Epub 2017 Oct 19.

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

Pilar Peris  1 , Xavier Filella  2 , Ana Monegal  3 , Nuria Guañabens  4 , Laura Foj  5 , María Bonet  6 , Dolors Boquet  7 , Enrique Casado  8 , Dacia Cerdá  9 , Alba Erra  10 , Carmen Gómez-Vaquero  11 , Silvia Martínez  12 , Nuria Montalá  13 , Concepción Pittarch  14 , Eduardo Kanterewicz  15 , Miquel Sala  16 , Xavier Suris  17 , Josep L Carrasco  18 ; LabOscat study group
Affiliations

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

Pilar Peris et al. Clin Chim Acta. 2017 Dec.

Abstract

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).

Methods: 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).

Results: 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.

Conclusions: 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.

Keywords: Bioavailable vitamin D; Free vitamin D; Vitamin D binding protein.

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