A competitive protein binding assay for plasma 25-hydroxyvitamin D3 in normal children.

In order to determine the plasma level of 25-hydroxyvitamin D3 (25-OH-D3) in Japanese children by the competitive protein binding assay, we studied binding proteins in D-deficient rats, and obtained the following results. 1) By using serum and kidney cytosol of D-deficient rats, we could obtain a standard curve with high sensitivity and accuracy which enabled us to determine the 25-OH-D3 level ranging from 0.5 to 5.0 ng. 2) The plasma 25-OH-D3 level was found to be 21.6+/-10.1 ng/ml (n=17) in healthy infants and children from 1 to 15 year-old, and 11.4+/-8.6 ng/ml (n=27) in mature neonates up to 2 days after delivery.