Anticardiolipin

Anticardiolipin antibodies are autoantibodies directed against cardiolipin, often in complex with beta-2 glycoprotein I, and belong to the family of antiphospholipid antibodies (along with lupus anticoagulant and anti-beta-2 glycoprotein I antibodies). Pathophysiologically, these antibodies interfere with the coagulation cascade, promote platelet activation, and inhibit protein C/protein S pathways, leading to a hypercoagulable state manifested by arterial and venous thromboses (e.g., DVT, stroke, pulmonary embolism) and pregnancy complications (recurrent miscarriages, especially after 10 weeks, preeclampsia, placental insufficiency). Clinically, anticardiolipin antibodies are one of the three laboratory criteria (along with lupus anticoagulant and anti-beta-2 glycoprotein I) used to diagnose antiphospholipid syndrome, which requires at least one clinical criterion (thrombosis or pregnancy morbidity) plus a positive lab test on two occasions at least 12 weeks apart. APS can be primary or secondary (often associated with SLE—up to 30-40% of SLE patients have aCL). A classic USMLE association is the paradox of a prolonged PTT in vitro (due to lupus anticoagulant, which is often co-present) but a prothrombotic state in vivo. Additionally, anticardiolipin antibodies cross-react with the antigens used in VDRL/RPR testing, causing a biologic false-positive syphilis test—an important board-relevant teaching point. Treatment involves long-term anticoagulation (typically warfarin, not DOACs, especially in triple-positive or high-risk APS) for those with thrombotic events, and low-dose aspirin plus heparin during pregnancy to prevent complications. Monitoring involves testing for antibody persistence over time since transient elevations can occur with infections and do not necessarily indicate APS.