Which laboratory test most directly assesses the extrinsic coagulation pathway?

• A. PT/INR
• B. aPTT
• C. D-dimer
• D. Platelet count

Answer: (A)

The test that directly evaluates the extrinsic pathway is PT/INR because it uses tissue factor to initiate clotting, and the resulting clotting time mainly reflects the activity of the extrinsic pathway and the downstream common pathway. Factor VII, a key component of the extrinsic pathway, has a short half-life and makes PT particularly sensitive to deficiencies (including vitamin K–related deficiencies or liver disease affecting these factors).

In contrast, the aPTT probes the intrinsic pathway (factors VIII, IX, XI, XII) plus the common pathway, so it’s not the best measure of the extrinsic cascade. D-dimer indicates fibrin degradation after clot formation and is a marker of fibrinolysis rather than initiation of coagulation. Platelet count relates to primary hemostasis and the number of platelets, not the coagulation cascade itself.