Competitive enzyme-linked immunoassay for Factor VIII antigen.

We describe a competitive enzyme-linked immunoassay for Factor VIII antigen. Binding of anti-factor VIII to solid-phase Factor VIII antigen is competitively inhibited by the free factor VIII antigen that is to be measured. The amount of anti-Factor VIII bound to solid-phase VIII is measured by applying in sequence a heterologous bridging antibody and a soluble antibody/enzyme immune complex. The soluble complex used was rabbit antiperoxidase/horseradish peroxidase. Peroxidase activity is inversely proportional to the Factor VIII antigen concentration in the original test plasma and is measured spectrophotometrically. The assay can be performed in as little as 4 h with only a microtiter plate, antisera, antigen, and a spectrophotometer. It is sensitive to 0.05 units of Factor VIII antigen per milliliter, and reproducibility, linearity, and normal range are similar to those reported for other techniques.