aPTT test assesses which pathways?

• A. Common pathway only
• B. Extrinsic pathway and intrinsic pathway
• C. Extrinsic pathway and the common pathway
• D. Intrinsic pathway and the common pathway

Answer: (D)

The main concept here is which parts of the coagulation cascade the aPTT is testing. Activated partial thromboplastin time measures the intrinsic pathway and the common pathway. The test uses an activator that starts the intrinsic (contact) factors, beginning with XII, and then proceeds through XI, IX, and VIII, before the cascade reaches the common pathway with factors X, V, II (prothrombin), and fibrinogen that form the clot. Because it depends on these internal factors and the shared downstream steps, it will be prolonged if there are deficiencies or inhibitors in the intrinsic or common pathways. The extrinsic pathway, which hinges on tissue factor and factor VII, is not activated by this test, which is why PT (not aPTT) assesses extrinsic plus common pathways.