Which coagulation pathway is primarily assessed by the aPTT?

Which coagulation pathway is primarily assessed by the aPTT?

• A. Extrinsic
• B. Intrinsic
• C. Common
• D. Fibrinolysis

Answer: (B)

Activated partial thromboplastin time (aPTT) probes the cascade triggered by contact activation to evaluate the intrinsic pathway. The test uses an intrinsic surface activator and phospholipid to start the reactions and then measures the time to fibrin formation. It is most sensitive to deficiencies or inhibitors of factors VIII, IX, XI, and XII, with contribution from the common pathway factors X, V, II, and I. Because of this setup, aPTT is the main test for the intrinsic pathway (and is also used to monitor unfractionated heparin therapy). In contrast, the extrinsic pathway is assessed by PT, which relies on tissue factor to activate factor VII, and fibrinolysis is tested by other measures.

Activated partial thromboplastin time (aPTT) probes the cascade triggered by contact activation to evaluate the intrinsic pathway. The test uses an intrinsic surface activator and phospholipid to start the reactions and then measures the time to fibrin formation. It is most sensitive to deficiencies or inhibitors of factors VIII, IX, XI, and XII, with contribution from the common pathway factors X, V, II, and I. Because of this setup, aPTT is the main test for the intrinsic pathway (and is also used to monitor unfractionated heparin therapy). In contrast, the extrinsic pathway is assessed by PT, which relies on tissue factor to activate factor VII, and fibrinolysis is tested by other measures.