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

ARTICLE. Peris P, Filella X, Monegal A, Guañabens N, Foj L, Bonet M, Kanterewicz E, Sala M, Suris X, Carrasco JL; LabOscat study group. (2017). Concordance between direct and indirect measurements of free 25-OH vitamin D. Clinica Chimica Acta, 475, 169–171. https://doi.org/10.1016/j.cca.2017.10.016

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 analyze 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 analyzed 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-OHD_F in these subjects were 5.27±2.5 and 3.83±1.01pg/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.