Vitamin D Binding Protein Assay

Dr Chris Pitt, Biochemistry

Vitamin D is stored in adipose tissue and enters the circulation bound to vit D binding protein (DBP). In the liver, Vit D is hydroxylated to 25-OH Vit D which also circulates bound to DBP. Physiologically, the most important fraction is that which is free in circulation and accessible to cells. This unbound fraction can be established by measuring total Vit D and the binding capacity of DBP.

The department wishes to develop the relevance and accuracy of its Vitamin D service therefore we intend to investigate the use of a DBP assay. Four options exist. The first is a nephelometric immunoassay. This is incompatible with our current autoanalyser platform. The second method is ultrafiltration isodialysis. This involves an 80 minute centrifugation step with radioactive tracer. The third method is a radioimmunoassay in which a pre-treatment step to analyse the Vit D binding capacity is integrated into a radioimmunoassay for VitD. The final option is to design our own in-house ELISA.