Which factor is primarily assessed by the PT assay?

• A. Factor VII primarily determines the PT, with additional reflection of factors II, V, and X
• B. Factor VIII determines the PT
• C. Factor VII exclusively determines the PT
• D. Factor IX determines the PT

Answer: (A)

The main concept tested here is what the prothrombin time (PT) measures. PT assesses the extrinsic pathway of coagulation and the downstream common pathway. The tissue factor–VIIa complex is the key initiator of the extrinsic pathway, so Factor VII activity largely determines the PT result. Because the extrinsic pathway ultimately feeds into the common pathway, the activities of the downstream factors II (prothrombin), V, and X also influence the PT, though to a lesser extent than Factor VII. This is why a prolonged PT most often points to issues with Factor VII, while still reflecting the status of II, V, and X. Factors VIII and IX belong to the intrinsic pathway and are more closely linked to the aPTT than the PT, so they do not primarily determine the PT.