Patent ID: 8133694

Claim:
A method of measuring the amount of 1α,25-dihydroxy vitamin D in human serum using a competitive protein binding assay, comprising: i) separating 25-hydroxy vitamin D from 1α,25-dihydroxy vitamin D by binding 1α,25-dihydroxy vitamin D in a sample of the human serum to a material that binds 1α,25-dihydroxy vitamin D and eluting 1α,25-dihydroxy vitamin D from said material to provide a measurement sample, ii) measuring the displacement of a vitamin D derivative of formula (I) from an antibody that specifically binds 1α,25-dihydroxy vitamin D by adding an amount of the measurement sample to an amount of the antibody having the vitamin D derivative of formula (I) bound thereto, wherein: R represents a 25-hydroxylated side-group of vitamin D 2 or of vitamin D 3 , and Y represents hydroxy; and iii) correlating the measurement of displacement of the vitamin D derivative of formula (I) from said antibody by 1α,25 dihydroxy-vitamin D present in the measurement sample to a measurement of displacement of the vitamin D derivative of formula (I) from the antibody by a known quantity of the 1α,25-dihydroxy vitamin D to determine the amount of 1α,25-dihydroxy vitamin D in the sample.