Factor IX activity is determined utilizing an aPTT-based one-stage clotting time assay. Factor IX-depleted plasma is used as the substrate, and the clotting time with the patient plasma is compared to the clotting time of normal pooled plasma. Patients with congenital deficiency of Factor IX are said to be suffering from a bleeding disorder: Hemophilia B, which is characterized by a tendency to bleed excessively due to the impaired blood clotting process.