Factor IX, also known as Christmas factor, is a clotting protein essential for the blood coagulation process. It is specifically involved in the intrinsic pathway of coagulation, helping to activate Factor X, which is necessary for the formation of a blood clot. A deficiency in Factor IX causes Hemophilia B, a genetic disorder characterized by difficulty in blood clotting and a tendency to bleed excessively, particularly after injury or surgery.

The Factor IX test measures the activity or level of this protein in the blood. Low levels of Factor IX confirm the diagnosis of Hemophilia B, a hereditary condition that primarily affects males. This test is used to diagnose Hemophilia B, monitor the severity of the disorder, and assess the effectiveness of treatments such as Factor IX replacement therapy.