Immunoradiometric assay of factor VIII related antigen, with observations in 32 patients with von Willebrand's disease

A solid phase non competitive immunoradiometric assay (IRMA) has been developed which allows measurement of factor VIII related antigen (VIIIR:AG) levels in normal plasma as low as 2.5x10^-4 U/ml. The assay is based on the extraction of VIIIR:AG from test plasma by means of polystyrene tubes coated with a specific unlabelled anti VIIIR:AG rabbit antiserum and subsequent labelling of the extracted antigen with ¹²⁵I labelled anti VIIIR:AG rabbit IgG. The amount of radioactivity retained by the tubes (percentage bound) is proportional to VIIIR:AG concentration and a linear correlation is observed when logs of plasma dilutions (from 1 in 300 to 1 in 4000) are plotted against logits of percentage bound. The IRMA is technically simple, specific and reproducible. In 32 normal persons there was a highly significant correlation (r = 0.94) between IRMA and rocket electro immunodiffusion assay (EIA) of VIIIR:AG plasma levels using the same antibody. IRMA has also been used to investigate 32 patients with von Willebrand's disease (vWd) in whom VIIIR:AG plasma levels were below the lowest measurable concentration using EIA (-4 U/ml and was extremely low in the remaining 6 (1.3x10^-2 to 4x10^-3 U/ml). In 15 patients the disease was familial (being transmitted as an autosomal dominant trait) and of moderate clinical severity; the IRMA or VIIIR:AG ranged between 1.2x10^-1 and 4x10^-2 U/ml. The assay thus appears a useful tool in the investigation of vWd and helps to identify two different types of the disease.