What is the only coagulation test that is sensitive to Factor VII deficiency?

The prothrombin time (PT) is the only coagulation test that is sensitive to Factor VII deficiency. This test specifically assesses the extrinsic pathway of coagulation, where Factor VII plays a pivotal role. It is triggered by the tissue factor released from damaged tissues, leading to the activation of Factor VII, which, in turn, activates Factor X. A deficiency in Factor VII will result in a prolonged PT, indicating problems with the extrinsic pathway.

In contrast, activated partial thromboplastin time evaluates the intrinsic pathway and is not affected by Factor VII levels. Thrombin time primarily assesses the conversion of fibrinogen to fibrin and, therefore, is not related to Factor VII deficiency. The fibrinogen test measures the levels of fibrinogen in the plasma and does not directly assess any specific coagulation factor involved in the extrinsic pathway. Thus, in the context of Factor VII deficiency, the prothrombin time provides the crucial sensitivity necessary to indicate such a deficiency.