Immunologic studies of factor IX (Christmas factor). II. Immunoradiometric assay of factor IX antigen.

A solid-phase two-site immunoradiometric assay has been developed which measures factor IX antigen levels as low as 0.0004 u per ml of plasma. In normal individuals, the factor IX antigen level correlated with the factor IX procoagulant level. In haemophilia B, 14 patients had markedly reduced antigen levels (less than 0.06 u/ml) and five had normal levels (greater than 0.60 u/ml).