Factor XI Deficiency

Laboratory Evaluation

Individuals who present with abnormal bleeding after surgery or injury must be fully evaluated with a coagulation profile, including prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen levels. A complete blood count is performed to exclude thrombocytopenia. Other causes of bleeding that may co-exist, particularly von Willebrand disease and platelet function disorders, require further specific testing. A co-existing disorder can explain some underlying variability in the bleeding tendency with FXI deficiency but is often not the full answer.

A prolonged aPTT is investigated by

a)      Performing mixing studies with normal plasma to exclude the presence of an inhibitor

b)      Performing specific factor assays of the relevant factors (VIII, IX, XI and XII) when the mixing study points to an underlying factor deficiency. It should be noted that a normal aPTT does not exclude an intrinsic factor deficiency. The sensitivity of aPTT reagents to FXI deficiency is variable; therefore, in the presence of bleeding symptoms and a normal aPTT an evaluation of intrinsic factor assays is mandatory.

c)      FXI deficiency must be differentiated from other causes of a prolonged aPTT when the diagnosis results from abnormal pre-operative screening tests, as the management of the different deficiencies varies. For example, isolated FXII deficiency requires no replacement therapy or correction prior to surgery, as FXII deficiency is not associated with an increased bleeding risk.

It should be noted that the factor XI level bears very little relationship to bleeding risk. See section above describing recent work with the thrombin generation test.