Historically, vitamin D was measured by competitive binding methods (RIA) and high-performance liquid chromatography (HPLC). A commonly used RIA kit, developed by DiaSorin S.p.A (Saluggia, Italy), was the method used by many reference laboratories and is considered the historical standard. This method has been used to establish reference ranges during the past decade. The DiaSorin 25-hydroxy vitamin D assay is a 2-step procedure that involves a rapid extraction of 25-hydroxy vitamin D and other hydroxylated metabolites from serum or plasma, followed by a competitive RIA procedure using an antibody with specificity for 25-hydroxy vitamin D.
