Antiphospholipid antibodies directed against phospholipid interfere with coagulation testing by binding to which molecule?

• A. Factor V
• B. Factor VIII
• C. Phospholipid
• D. Calcium

Answer: (C)

Antiphospholipid antibodies cause interference in coagulation tests by binding to the phospholipid surface that these tests rely on. In many clotting assays, a phospholipid-containing reagent provides the surface where coagulation complexes assemble (with calcium and the vitamin K–dependent factors). The antibodies bind to phospholipid–protein complexes on this surface, blocking normal assembly of the enzyme complexes and prolonging the clotting time. That’s why the molecule involved is phospholipid. The antibodies aren’t primarily directed at factors like V or VIII or at calcium themselves, but at the phospholipid surface used in the test reagents, leading to erroneous prolongation in vitro.