Features of a Factor XI inhibitor:
(1) prolonged aPTT and normal PT
(2) deficiency in Factor XI activity
(3) failure of the prolonged aPTT to correct with addition of normal plasma
(4) both females and males affected
(5) a mild to moderate bleeding disorder may be present
Patients at risk for developing a Factor XI inhibitor:
(1) severe hereditary Factor XI deficiency
(2) SLE or other autoimmune disorder
A patient with hereditary Factor XI deficiency may show a worsening in clinical bleeding.
A patient with an acquired inhibitor not related to hereditary Factor XI deficiency will have a previous history of normal aPTT and hopefully normal hemostasis.
Differential diagnosis:
(1) other inhibitors to coagulation factors (lupus anticoagulant, Factor VIII inhibitor, Factor IX inhibitor, etc.)
(2) Noonan's syndrome (associated with a variety of coagulation factor abnormalities, including Factor XI deficiency)
(3) hereditary Factor XI deficiency without an inhibitor
