[Method of determining factor IX activity]

Modification of the method for factor IX activity determination was proposed. A mixture of chicken plasma and human plasma was used as factor IX-deficient substrate plasma. Moreover determinations were performed in the lack of kaolin-activator of factor XII. This allowed simplifying the assay performance. The described modification may be used to determine factor IX activity in its concentrates with the aim of diagnosis of haemophilia B and control of the disease treatment.