Differential diagnosis involves ruling out other causes for lengthening of APTT associated with decreased activity of factor IX.
Acquired haemophilia B is associated with the presence in non-haemophiliac subjects of antibodies directed against factor IX. This is a rare disease that can occur in particular in women during or at the end of pregnancy, in patients with autoimmune disease, and in elderly subjects.
The most common clinical sign is the presence of marked haematomas. Since the disease may be life-threatening, appropriate therapy must be given quickly.
Diagnosis involves screening for a specific inhibitor directed against factor IX using the Bethesda method or the Nijmegen method. These inhibitors are also seen more frequently (10 to 15% of patients) in haemophiliac subjects receiving replacement therapy containing factor IX, against which they are immunised due to the development of antibodies directed against factor IX.
Since the synthesis of factor IX is dependent upon vitamin K, levels of factor IX are low in the event of vitamin K deficiency or treatment with vitamin K antagonists.
In this event, the levels of other vitamin K-dependent coagulation factors (II, VII, X) are also low, as are proteins S and C.