The aPTT is a screening test for the laboratory evaluation of inherited and acquired deficiencies in which of the following?

• A. Extrinsic pathway of the coagulation cascade
• B. Intrinsic pathway of the coagulation cascade
• C. Platelets
• D. Vascular system

Answer: (B)

The test is focused on the intrinsic pathway of coagulation. It triggers the cascade using contact activation and phospholipid, so it relies on factors XII, XI, IX, and VIII, with the results reflecting anything affecting those intrinsic pathway factors (and the downstream common pathway factors II, V, X, and fibrinogen). When any intrinsic pathway deficiency or inhibitor is present, aPTT becomes prolonged, which is why it’s used to screen for inherited conditions like hemophilia or acquired factor deficiencies in this pathway. Problems in the extrinsic pathway, tested by prothrombin time, don’t typically prolong the aPTT. Platelet issues and vascular abnormalities influence bleeding but aren’t what the aPTT directly screens for.