Discrepancy between serum concentrations of 1,25-dihydroxyvitamin D metabolites measured by radio-immunoassay and thymus radioreceptor assay during vitamin D₂ treatment

Abstract

A radio-immunoassay (RIA) for determination of 1,25-dihydroxycholecalciferol (l,25(OH)₂D₃) and 1,25-dihydroxyergocalciferol (1,25(OH)₂D₂) in serum was compared with a competitive protein binding assay using calf thymus receptor to clarify the comparability of measurements in subjects treated with either vitamin D₂ or vitamin D₃ (4000 IU per day for 8 weeks). Before and during treatment with vitamin D₃ the two assay techniques were concordant, but during vitamin D₂ treatment the serum concentrations of 1,25(OH)₂D₂ and 1,25(OH)₂D₃ were higher when measured by RIA. The study demonstrates that during vitamin D₂ treatment unknown compounds, recognized only by the antiserum, co-elute with 1,25(OH)₂D₂ and 1,25(OH)₂D₃ in high pressure liquid chromatography. It can be concluded that the radio-immunoassay used here cannot replace the calf thymus receptor assay.

Key Words:

• antiserum
• 1,25(OH)₂D₂
• 1,25(OH)₂D₃