What are antiphospholipid (APL) antibodies?

Updated: Jul 22, 2021
  • Author: Tarek Hammad, MD; Chief Editor: Eric B Staros, MD  more...
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Antiphospholipid (APL) antibodies are group of heterogeneous antibodies directed against epitopes on plasma proteins that are uncovered by binding of these proteins to anionic phospholipids on plasma membranes. These antibodies can be detected with lupus anticoagulant (LAC), anticardiolipin antibodies (ACL), and anti-β2 -glycoprotein I antibodies. [6]

LAC is a functional assay that is used to detect APL antibodies that have anticoagulation activity in vitro. LAC is a misnomer, since it is clinically associated with clotting tendency rather than anticoagulation activity, and only 50% of people with LAC meet the criteria for systemic lupus erythematosus (SLE).

The clinical significance of the above antibodies is their association with thrombosis; once an individual is found to have one of the above antibodies with vascular thrombosis and/or pregnancy morbidity, antiphospholipid antibody syndrome (APS) is diagnosed. [7, 8, 9, 10] APS occurs either as a primary disease or in the setting of another disease such as SLE (so-called secondary APS).

A study by Pablo et al indicated that in persons who carry APL antibodies but do not meet the clinical criteria for APS, independent risk factors for the development of thrombosis include smoking, hypertension, thrombocytopenia, and triple APL antibody positivity. [11]

In addition of the above APL antibodies, other APL antibodies exist, such as prothrombin, annexin V, phosphatidylserine, phosphatidylinositol, and phosphatidylethanolamine, but their clinical significance remains elusive and experience is limited; thus, none is yet part of the diagnostic criteria of APS.

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