Technical problems in the assay of serum 25-hydroxyvitamin D.

Various methods to enhance the solubility of 25-hydroxyvitamin D in the aqueous buffer used for competitive protein binding assay are compared. The problem of non-specific binding caused by the addition of various substances to the buffer to prevent loss of 25-hydroxyvitamin D from the reaction mixture is also evaluated. Bovine albumin at the concentration of 0.2-0.4 g % seems to represent the best solution of the above problems, since it exerts a good solubilizing effect on 25-hydroxyvitamin D with low non-specific binding counts.