Measurement of antihaemophilic factor A antigen (VII:CAg) with a solid phase immunoradiometric method based on homologous non-haemophilic antibodies.

Antihaemophilic-factor-A-antibodies, which had spontaneously arisen in 2 patients, were used to develop an immunoradiometric method for measurement of antihaemophilic factor A antigen (VIII:CAg). 13 patients with severe haemophilia A had VII:CAg below the limit of detection (0.01 U/ml). Patients with moderate and mild haemophilia A either had VII:CAg roughly equal to factor VIII clotting activity (VIII:C) or a not detectable VII:CAg, suggesting 2 different molecular mechanisms in moderate and mild haemophilia A. VIII:CAg could be detected in serum but in lower amounts than in plasma. In 2 patients with von Willebrand's disease VIII:CAg equalled VII:C. The post-transfusional retarded increase of VII:C in 1 patient with von Willebrand's disease was accompanied by a slight increase in VIII:CAg. Fetal plasma contained measurable amounts of VII:CAg.