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

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.