Which test primarily assesses the extrinsic coagulation pathway?

• A. aPTT
• B. PT
• C. D-dimer
• D. Bleeding time

Answer: (B)

The main concept is how to assess the extrinsic pathway of coagulation. The extrinsic pathway is activated by tissue factor exposure and involves factor VII to kick off the cascade that leads to thrombin formation. The prothrombin time uses tissue factor (thromboplastin) and calcium to measure how long plasma takes to clot, and it is especially sensitive to deficiencies in factor VII (which has a short half-life) as well as other vitamin K–dependent factors II, VII, and X. Because of this sensitivity, PT lengthens when the extrinsic pathway is impaired, making it the test that best reflects this pathway. In contrast, the aPTT measures the intrinsic pathway (factors XII, XI, IX, VIII) plus the common pathway; D-dimer reflects fibrin degradation and is not a direct measure of pathway function; and bleeding time assesses platelet function and vessel integrity rather than coagulation factors. So the prothrombin time is the test that primarily assesses the extrinsic coagulation pathway.