Immunonephelometric quantification of serum vitamin D-binding globulin.

Rapid quantification of serum vitamin D-binding globulin (VDBG) is described. An immunonephelometric method was developed and compared with the conventional immunodiffusion method for VDBG. A correlation coefficient of 0.896 (P less than 0.001) was found with a regression line y = 0.965x + 0.011 (n = 23). The within-assay and day-to-day coefficient of variation of the immunonephelometric method was 6 and 11% at 0.150 g/l (low VDBG concentration), 5 and 7% at 0.230 g/l (normal), and 4 and 8% at 0.570 g/l (high). Levels of VDBG and total 25-hydroxyvitamin D (25(OH)D, 25(OH)D2 + 25(OH)D3) were analysed from 33 serum samples obtained in April-May 1980 from 14-month-old children undergoing vitamin D substitution. 25(OH)D was extracted from 0.1-0.5 ml serum followed by semipreparative silica column chromatography and final quantification by a competitive protein-binding method. The VDBG mean value was 0.308 +/- 0.068 g/l (SD) and 25(OH)D had a mean value of 121 +/- 70 nmol/l (SD). There was no significant correlation between the levels of VDBG and 25(OH)D. The results of this study in general agreed well with other findings.