Detection of factor IX inhibitors by immunoradiometric assay.

An immunoradiometric assay (IRMA) for the determination of factor IX inhibitors in haemophilia B was developed. The assay was based on competitive binding between radiolabelled anti-IX and inhibitors in the test material of immobilized IX:C. 5 haemophiliacs with known inhibitors were investigated with the new method and with a conventional neutralization test. An inhibitor titre as low as 5 X 10(-4) U/ml could be measured by IRMA, showing it to be about 500 times as sensitive as the conventional neutralization assay used, and in 2 cases the inhibitors could only be detected by IRMA. The new method is thus useful for the investigation of patients with low inhibitor titres, or when inhibitors with divergent properties are suspected.