An enzyme immunoassay (ELISA) for the quantitation of human factor VIII coagulant antigen (VIII:CAg)

For the purposes of prenatal diagnosis and carrier detection of haemophilia A, an enzyme linked immunosorbent assay (ELISA) was developed for the quantitation of plasma Factor VIII coagulant antigen (VIII:CAg). It is based upon a human antibody. Results are compared to assays for Factor VIII coagulant activity (VIII:C) and Factor VIII related antigen (VIIIR:Ag) in plasma from 30 normal female individuals, 5 fetuses from mothers normal with respect to bleeding status, 10 obligate carriers of haemophilia A, 10 patients with haemophilia A, 5 with von Willebrand's disease, and 5 with haemophilia B. The ELISA developed is simpler than previously published VIII:CAg methods owing to its use of total IgG instead of immunologically affinity-purified antibodies. It is specific (as judged from clinical results), sensitive (detection limit: 0.005 units/ml), and sufficiently precise (between-assay coefficient of variation: 11%) for the purposes mentioned. The coefficient of correlation between VIII:CAg and VIII:C results is 0.86. The introduction of ELISA for quantitating VIII:CAg represents an advantage as compared to existing immunoradiometric assays (IRMA) mainly due to the stable and non-radioactive reagents used in the ELISA.