**3.3.1.2 Inhibitors of hemostasis**

The hemostatic process is tightly regulated by specific inhibitors that act on coagulation factors and on the factors of primary hemostasis. Functionally most important are tissue factor pathway inhibitor, the PC system, antithrombin (AT) and the VWF cleaving protease ADAMTS13. Clinically, to date only the latter three are important. Involvement of the coagulation inhibitors AT, PC and Protein S (PS) is rare with a prevalence in unselected patients with thrombosis of 0.019 for AT, 0.037 for PC, and 0.023 for PS deficiency.(Pabinger & Schneider, 1996; Koster et al, 1995) Recently, severe deficiency of ADAMTS13 has been identified as the causative factor of the rare TTP in most TTP patients (Lammle et al, 2005).
