Which test is most appropriate to assess the intrinsic pathway of coagulation?

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

Answer: (B)

Activated partial thromboplastin time targets the intrinsic pathway. It initiates coagulation through contact factors (XII, XI, IX, VIII) with an activator, then proceeds through the common pathway to form a clot. The time it takes reflects how well the intrinsic factors are functioning, making this test the best choice for assessing the intrinsic pathway.

Prothrombin time, on the other hand, activates the extrinsic pathway using tissue factor and measures the shared downstream steps, so it’s more about the extrinsic pathway. Thrombin time examines the conversion of fibrinogen to fibrin and is sensitive to fibrinogen abnormalities and thrombin inhibitors, not the pathway itself. Reptilase time checks fibrinogen function in a way that bypasses thrombin, so it also doesn’t specifically evaluate the intrinsic coagulation cascade.