Abstract
A solid-phase 2-site immunoradiometric assay was developed which measures factor IX antigen levels as low as 0.0004 U[units]/ml of plasma. In normal individuals the factor IX antigen level correlated with the factor IX procoagulant level. In hemophilia B, 14 patients had markedly reduced antigen levels (< 0.06 U/ml) and 5 had normal levels (> 0.60 U/ml).