How do antiphospholipid antibodies correlate with thrombosis risk despite prolongation of coagulation assays?

• A. They cause in vitro prolongation by phospholipid interference but in vivo contribute to a hypercoagulable state.
• B. They shorten coagulation tests in vitro.
• C. They are associated with decreased thrombosis risk.
• D. They have no effect on thrombosis risk.

Answer: (D)

Antiphospholipid antibodies create a paradox: they cause prolongation of phospholipid-dependent coagulation assays in the lab, yet in the body they promote a hypercoagulable state that increases the risk of thrombosis. In the laboratory, these antibodies bind to phospholipid–protein complexes on test surfaces, interfering with the assembly of coagulation enzyme complexes that require phospholipids, which lengthens clotting times such as the lupus anticoagulant–type tests. In vivo, they drive thrombosis by activating endothelial cells and platelets, promoting tissue factor expression, disrupting natural anticoagulant pathways (like protein C/S), and altering the phospholipid shield on cell membranes, all of which enhances thrombin generation and clot formation. So the best understanding is that antiphospholipid antibodies prolong coagulation assays in vitro because of phospholipid interference, but they are associated with a real, increased risk of thrombosis in patients. The other statements mischaracterize either the test behavior or the clinical risk, making them less accurate.