For Research Use Only. Not for Use in Diagnostic Procedures. Two-site ELISA method for the measurement of human factor V antigen. Assay designed with a monoclonal antibody specific for the factor V heavy chain, for coating the microplate (immunocapture), and a horse anti-factor V polyclonal antibody, coupled to peroxidase, for revelation. This assay preferentially measures native factor V, and is also reactive with factor Va, but with a decreased reactivity.