By virtue of a severely prolonged aPTT with a normal thromboplastin time (prothrombin time) and a normal thrombin time, severe FXII deficiency has been diagnosed in a woman without a bleeding diathesis or a history of thromboembolic complications. A deficiency of a factor of the contact activation system (FXII, prekallikrein, high molecular weight kininogen) is usually diagnosed during routine coagulation tests demonstrating a prolonged aPTT. The severe and partial deficiency of FXII, of prekallikrein or high molecular weight kininogen is not associated with a bleeding tendency. In contrast, severely factor XI deficient subjects may suffer from a mild hemorrhagic diathesis, whereas FVIII deficiency (hemophilia A, autoimmune "hemophilia", von Willebrand disease) and FIX deficiency (hemophilia B) are associated with a bleeding tendency of varying severity, depending on the clotting activity of FVIII or FIX, respectively. An isolated prolongation of the aPTT due to a lupus anticoagulant, however, is frequently associated with arterial and/or venous thrombosis. Therefore, in case of a prolongation of the aPTT, its cause has to be determined.