Which test evaluates the extrinsic coagulation pathway?

• A. Prothrombin time
• B. Activated partial thromboplastin time
• C. Hemostatic assay
• D. Thrombin time

Answer: (A)

Prothrombin time evaluates the extrinsic coagulation pathway because it uses tissue factor (thromboplastin) and calcium to trigger clotting in plasma. This setup specifically tests how well the extrinsic pathway—driven by tissue factor and factor VII, along with vitamin K–dependent factors II, VII, and X feeding into the common pathway—functions. A prolonged time indicates issues with this route. In contrast, the activated partial thromboplastin time uses an intrinsic pathway activator and gauges the intrinsic (and common) pathway, thrombin time assesses the final conversion of fibrinogen to fibrin, and a general hemostatic assay lacks the specificity to pinpoint the extrinsic pathway.