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

Abstract At present, data comparing the quantification methods for measurement of free vitamin D (direct assay [direct 25-OHD F ] and estimated by calculation [calculated 25-OHD F ]), are scarce. The aim of this study was to analyse the concordance between these two methods of 25-OHD F analysis (direct vs. calculated). Methods Serum values of total 25-OHD (25-OHD T ), vitamin D binding protein (DBP) (by R&D Systems ELISA), calculated 25-OHD F (by DBP, albumin and 25-OHD T ) and direct 25-OHD F (by DIAsource ELISA) were analysed in 173 healthy women (aged 35–45 years). 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-OHD F in these subjects were 5.27 ± 2.5 and 3.83 ± 1.01 pg/mL, respectively. We found significantly lower values of 25-OHD F 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-OHD F levels were concordant. Conclusions This study shows that the concordance between these two methods of 25-OHD F analysis is low and has a concentration dependent bias. Further studies are necessary to clarify the reference values and the indications for 25-OHD F measurement.