Two cases of Factor XI deficiency: Use of Thrombin Generation Assays (TGA) to detect a non-bleeding phenotype.

Factor XI deficiency is a rare disorder of hemostasis. Previously also known as "hemophilia C", this defect has been regarded as a risk factor for bleeding. However, it has been known for long that bleeding tendency and severity of bleeding are not related to the residual factor XI activity in symptomatic patients. Moreover, a large proportion of patients with even severe factor XI deficiency are clinically unremarkable and do not show any signs of abnormal bleeding. Here, we present two cases of factor XI deficiency with a non-bleeding phenotype. Adequate diagnostic work-up and evaluation of the bleeding risk are reported and discussed with focus on thrombin generation assays (TGA) for the prediction of bleeding in affected patients. This is of high relevance in affected patients, particularly in the context of surgery.