Simplified immunoradiometric assay for factor VIII coagulant antigen

Abstract

A simplified, noncompetitive, solid phase immunoradiometric assay was developed for the quantitation of factor VIII coagulant antigen (VIII:CAg)-the antigenic counterpart of FVIII coagulant activity (VIII:C). Both homologous and heterologous antibodies to human factor VIII (FVIII) were used in this assay. FVIII in a test sample was attached to immobilized, human IgG obtained from a polytransfused hemophilia A patient with a high titer antibody to VIII:C. The bound FVIII was then detected with rabbit 125I-IgG specific for human FVIII. The concentration of VIII:CAg correlated well with VIII:C levels in the plasma from normal donors (r = 0.84, n = 15). Homozygote von Willebrand''s disease patients had undetectable levels of VIII:CAg in their plasma. Patients with severe hemophilia A (VIII:C < 0.01 U/ml) could be divided into groups on the basis of the VIII:CAg levels, i.e., those having undetectable VIII:CAg and others with measurable VIII:CAg. VIII:CAg detected in normal serum was < 0.002 U/ml. In this assay the use of human antibody to FVIII is considerably decreased compared to other methods for VIII:CAg, and the time-consuming steps to immunopurify human anti-FVIII antibody are eliminated.