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

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

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.

MeSH terms

  • Adult
  • Calcifediol / blood*
  • Enzyme-Linked Immunosorbent Assay / standards*
  • Female
  • Humans
  • Middle Aged
  • Regression Analysis
  • Reproducibility of Results
  • Vitamin D Deficiency / blood*
  • Vitamin D Deficiency / diagnosis
  • Vitamin D-Binding Protein / blood

Substances

  • Vitamin D-Binding Protein
  • Calcifediol