A patient with SLE who had a prior DVT asks which test is most important to order to evaluate for antiphospholipid syndrome.

• A. Anti-lupus anticoagulant
• B. D-dimer
• C. FactorVLeiden
• D. Anti-histone

Answer: (A)

The main concept is identifying antiphospholipid syndrome in a patient with SLE who has had a thrombotic event. The lupus anticoagulant test is the most important single laboratory assessment for APS because it directly detects antiphospholipid antibodies that are strongly linked to thrombosis risk in APS. Although these antibodies prolong certain clotting tests in the lab (in vitro), they paradoxically predispose to clotting in the body. Testing with lupus anticoagulant–sensitive assays (and paired anticardiolipin and anti–β2-glycoprotein I antibodies) fulfills the laboratory criteria for APS and helps guide long-term management like extended anticoagulation. D-dimer is nonspecific and not diagnostic for APS, Factor V Leiden checks a different inherited thrombophilia, and anti-histone relates to drug-induced lupus rather than APS.