Which clotting factor is involved in clot formation in vitro but not in vivo?

Factor XIIa is involved in the intrinsic pathway of coagulation, which can be activated in vitro, such as in laboratory settings like the activated partial thromboplastin time (aPTT) test. In this context, Factor XII can initiate the coagulation cascade when in contact with negatively charged surfaces, promoting fibrin clot formation without the need for tissue factor.

In vivo, however, Factor XII plays a minimal role in hemostasis. The body primarily relies on tissue factor, which is not exposed during normal vascular conditions, and Factor VIIa in the extrinsic pathway to initiate clotting. Therefore, while Factor XIIa is crucial for clotting processes activated artificially in the lab, it does not contribute significantly to the physiological coagulation cascade that occurs naturally within the body.