What does aPTT assess?

• A. Intrinsic and common coagulation pathways; monitoring heparin therapy
• B. Extrinsic pathway
• C. Platelet aggregation
• D. Fibrinogen levels

Answer: (A)

Activated partial thromboplastin time (aPTT) measures how long it takes for plasma to clot after activating the contact factors and adding phospholipid. This test specifically evaluates the intrinsic pathway (factors XII, XI, IX, VIII) and the common pathway (factors X, V, II, I). Because heparin enhances antithrombin III’s inhibition of thrombin and other steps in these pathways, its presence lengthens the aPTT, so this test is commonly used to monitor unfractionated heparin therapy. It does not assess the extrinsic pathway (that’s what PT measures), nor does it directly test platelet function or fibrinogen levels (those require other tests such as PT, platelet aggregation studies, thrombin time, or specific fibrinogen assays).