Immunoradiometric assay of procoagulant factor-VIII antigen in plasma and serum and its reduction in haemophilia. Preliminary studies on adult and fetal blood.

An immunoradiometric assay (I.R.M.A.) for antihaemophilic factor (factor VIII) has been developed which is based on a human inhibitor to procoagulant factor VIII (FVIIIC) which arose in a polytransfused haemophiliac. The assay detected and measured the antigen related to FVIIIC (FVIIICAg) in normal plasma and serum but failed to detect any antigen in samples from five patients with severe haemophilia. In a sixth patient with severe haemophilia a non-parallel dose-response curve was obtained. FVIIICAg was also present in plasma and serum from six samples of cord blood and in two samples of fetal serum obtained after termination of pregnancy. The assay may, therefore, be used to measure FVIIICAg in small serum samples and provides a potential practical method for prenatal diagnosis of haemophilia.