Immunonephelometric assay of vitamin D-binding protein

Abstract

An automated immunonephelometric assay was developed to measure vitamin D-binding protein (DBP) in serum. The assay is described for the Behring Nephelometer System, which uses rabbit anti-DBP antiserum and purified human DBP. The detection limit is 0.05 g/L (0.86 μmol/L), and the working range is ≤1.60 g/L (27.59 μmol/L). Intra- and interassay CVs of 2.0% and 2.8-3.8% compare favorably with alternative methods. When results were compared with those from a immunoradiometric assay, the correlation coefficient was 0.976 (P <0.001), and the regression equation [y = 0.866 ± 0.085x + 0.05 (Syx = 0.042, n = 42)] identified a negative bias. Analysis indicated that both methods appeared to contribute equally to the bias. Although the assay was relatively free from analytical interference, falsely increased values were noted in severely lipemic specimens and in frozen specimens. Interference may be minimized by inclusion of Supplementary Precipitation reagent in a modified assay protocol. The range of concentrations expected in clinical samples was established from normal subjects [0.32-0.46 g/L (5.52-7.93 μmol/L), n = 28], pregnant subjects [0.51-0.70 g/L (8.79-12.07 μmol/L), n = 13], and subjects with liver diseases [0.12-0.33 g/L (2.07-5.69 μmol/L), n = 18].