What is the mechanism behind lupus anticoagulant prolonging the aPTT?

• A. Autoantibody against thrombin
• B. Factor VIII deficiency
• C. Excess fibrinogen degradation products
• D. In vitro phospholipid-dependent inhibitor

Answer: (D)

Lupus anticoagulant works by antibodies that target phospholipid–protein complexes, which interferes with phospholipid-dependent steps in the coagulation cascade when tested in the lab. In tests like the aPTT, clotting requires a phospholipid surface to assemble the enzyme complexes (such as the tenase and prothrombinase complexes). The lupus anticoagulant binds to these phospholipid surfaces and inhibits the reactions, so more time is needed to form a clot. This makes the aPTT appear prolonged in vitro, even though patients may be prone to clotting in vivo.

So the prolongation is due to an in vitro phospholipid-dependent inhibitor, produced by antiphospholipid antibodies. The other ideas don’t fit this mechanism: an autoantibody against thrombin would block thrombin directly, a factor VIII deficiency would cause a prolongation from a true factor deficiency, and fibrinogen degradation products affect clot formation in different ways but aren’t the specific phospholipid-dependent inhibitory effect seen with lupus anticoagulant.