[Immunological assay of substances of blood and vascular bed origin, and their future]

Recently in the clinical laboratories, many highly sensitive immunological measurements are performed using such techniques as radioimmunoassay, enzyme immunoassay, fluoroimmunoassay, and chemiluminescence immunoassay. However, the metabolic clearance rate of a very small amount of substance in the circulatory blood may participate intimately in its inactivation and/or trapping in a single passage through a certain organ. The difficulty of immunological assays for very small amounts of biologically active substances in blood and the possibility of developing an easier assay method are discussed.