Factor VIII Clotting Antigen (VIIICAg) in Haemophilia Measured by Two Immunoradiometric Assays (IRMA) using Different Antibodies, and the Measurement of Inhibitors to Procoagulant Factor VIII (VIIIC) by IRMA

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Abstract
Factor VIII clotting antigen (VIIICAg) was measured by immunoradiometric assay (IRMA) using 2 different antibodies. Both antibodies arose in polytransfused severe hemophiliacs and had similar titers against VIIIC. In 12 normal plasmas there was no significant difference in VIIICAg values obtained (VIIICAg (AbI) = VIIICAg (AbII)). In the majority of 15 severe hemophiliacs tested VIIICAg was undetectable by both antibodies. In 28 mild to moderate hemophiliacs VIIICAg (AbII) was significantly greater than VIIICAg (AbI) (P < 0.01) suggesting different antigenic determinants. The difference was small and does not affect diagnosis of hemophilia. A modified IRMA was used to measure anti VIIIC inhibitors by competition of the inhibitor with 125I labeled VIIICAg antibodies for common antigenic determinants. Using an inhibitor of 225 Bethesda units as a standard, results by IRMA of inhibitors in severe hemophiliacs were similar to those obtained by clotting assay, but with a sensitivity of 0.01 U/ml suggesting the possible use in the detection of weak inhibitors.