Antiphospholipid Syndrome

Updated: Nov 15, 2023
  • Author: Lauren Tesoriero, DO; Chief Editor: Herbert S Diamond, MD  more...
  • Print

Practice Essentials

Antiphospholipid syndrome (APS) is an acquired autoimmune disorder that manifests clinically as recurrent venous or arterial thrombosis and/or fetal loss. [1] Characteristic laboratory abnormalities in APS include persistently elevated levels of antibodies directed against membrane anionic phospholipids (ie, anticardiolipin [aCL] antibody, antiphosphatidylserine) or their associated plasma proteins, (predominantly beta-2 glycoprotein I [apolipoprotein H]); and/or evidence of a circulating anticoagulant. See Presentation and Workup.

Some patients have isolated APS, with no evidence of any definable associated disease. In other patients, APS occurs in association with systemic lupus erythematous (SLE) or another rheumatic or autoimmune disorder. Traditionally, these are referred to as primary or secondary APS, respectively. Although antiphospholipid (aPL) antibodies are clinically linked to APS, whether they are involved in the pathogenesis or are an epiphenomenon is unclear. Up to 5% of healthy individuals are known to have aPL antibodies.

The development of aPL antibodies has been described in association with thrombosis in patients with COVID-19, and in a cohort study from France, lupus anticoagulant was detected in a large percentage of patients with severe COVID-19. However, it is not yet known whether the aPL antibodies in these patients play a role in the thrombosis that occurs in COVID-19 cases, or merely represent an association. [2]

In general, treatment regimens for APS must be individualized according to the patient's current clinical status and history of thrombotic events. Asymptomatic individuals in whom blood tests show aPL antibodies do not require specific treatment. For patients with APS, prophylaxis is needed during surgery or hospitalization, as is management of any associated autoimmune disease. Low-dose aspirin is used widely in this setting; although its effectiveness remains unproven. For thrombosis, perform full anticoagulation with intravenous or subcutaneous heparin followed by warfarin therapy. See Treatment.

For discussion of APS in children, see Pediatric Antiphospholipid Antibody Syndrome. For discussion of obstetric APS, see Antiphospholipid Syndrome and Pregnancy.



In APS, the homeostatic regulation of blood coagulation is altered; however, the mechanisms of thrombosis are not well defined. One hypothesis postulates a defect in cellular apoptosis, which exposes membrane phospholipids to the binding of various plasma proteins, such as beta-2 glycoprotein I. Once bound, a phospholipid-protein complex is formed and a neoepitope is uncovered, which subsequently becomes the target of autoantibodies. Evidence suggests that oxidized beta-2 glycoprotein I is able to bind to and activate dendritic cells in a manner similar to activation triggered by Toll-like receptor 4 (TLR-4), which could amplify the production of autoantibodies. [1, 3]

Other proposed mechanisms for the hypercoagulable effect of aPL antibodies, which may or may not depend on beta-2 glycoprotein I, include the following:

  • Production of antibodies against coagulation factors, including prothrombin, protein C, protein S, and annexins
  • Activation of platelets to enhance endothelial adherence
  • Activation of vascular endothelium, which in turn facilitates the binding of platelets and monocytes
  • Reaction of antibodies to oxidized low-density lipoprotein, thus predisposing to atherosclerosis and myocardial infarction (MI)

Complement activation has been increasingly recognized as possibly having a significant role in the pathogenesis of APS. Failure of the normal complement-regulating mechanisms can result in uncontrolled complement activation, leading to complement-mediated direct cellular injury and thrombosis. [4] Evidence from murine models suggests that aPL-mediated complement activation may be a primary event in pregnancy loss. [3, 5] Rare germline variants in complement regulatory genes have been found in 60% of patients with catastrophic APS, compared with 21.8% of patients with thrombotic APS and 23.3% of normal controls. [6]

Clinically, the series of events that leads to hypercoagulability and recurrent thrombosis can affect virtually any organ system, including the following:

  • Peripheral venous system ( deep venous thrombosis [DVT])
  • Peripheral nervous system (peripheral neuropathy, including Guillain–Barré syndrome)
  • Hematologic (thrombocytopenia,  hemolytic anemia)
  • Obstetric (pregnancy loss, eclampsia)
  • Pulmonary ( pulmonary embolism [PE], pulmonary hypertension)
  • Dermatologic (livedo reticularis, purpura, infarcts/ulceration)
  • Cardiac (Libman-Sacks valvulopathy, MI, diastolic dysfunction)
  • Ocular (amaurosis, retinal thrombosis)
  • Adrenal (infarction/hemorrhage)
  • Musculoskeletal (avascular necrosis of bone)
  • Renal (thrombotic microangiopathy)

The kidney is a major target organ in APS. Nephropathy in APS is characterized by small-vessel vaso-occlusive lesions associated with fibrous intimal hyperplasia of interlobular arteries, recanalizing thrombi in arteries and arterioles, and focal atrophy. [7]

A “two-hit” theory has been proposed in which a second risk factor (age, hypertension, diabetes, obesity, smoking, pregnancy, surgery, infection, other genetic hypercoagulable state) incites the thrombotic effects of aPL in a patient with underlying genetic predisposition. [8]



APS is an autoimmune disorder of unknown cause. The search for possible triggers has uncovered a wide array of autoimmune or rheumatic diseases, infections, and drugs that are associated with the lupus anticoagulant or anticardiolipin antibodies. These associations may ultimately provide a clue to the etiology of APS.

A considerable percentage of persons with certain autoimmune or rheumatic diseases also have aPL antibodies. Common autoimmune or rheumatic diseases and the percentage of affected patients with aPL antibodies are as follows (note that these represent percentages of patients with aPL antibodies, rather than the clinical syndrome of APS [9] ):

  • SLE - 25-50%
  • Sjögren syndrome - 42%
  • Rheumatoid arthritis - 33%
  • Immune thrombocytopenia - 30%
  • Psoriatic arthritis - 28%
  • Systemic sclerosis - 25%
  • Mixed connective-tissue disease - 22%
  • Polymyalgia rheumatica or giant cell arteritis - 20%
  • Behçet syndrome - 20%
  • Autoimmune hemolytic anemia - Unknown

Infections associated with APS include the following [10] :

  • Syphilis
  • Hepatitis C 
  • HIV infection
  • Human T-cell lymphotrophic virus type 1 infection
  • Malaria
  • Bacterial septicemia

Drugs associated with APS include the following:

  • Cardiac - Procainamide, quinidine, propranolol, hydralazine
  • Neuroleptic or psychiatric - Phenytoin, chlorpromazine
  • Other - Interferon alfa, quinine, amoxicillin

In addition, certain vaccines have been associated with APS. For example, vaccination with tetanus toxoid may trigger the formation of antibodies that cross-react with beta-2 glycoprotein I, due to molecular mimicry between the two molecules. [11]

 Genetic predisposition may be involved, as follows:

  • Relatives of persons with known APS are more likely to have aPL antibodies. One study showed a 33% frequency.
  • An association has been found between aCL antibody and carriage of certain HLA genes, including  DRw53, DR7 (mostly in people of Hispanic origin), and  DR4 (mostly in Whites). [12]


The frequency of APS is likely similar in the United States and internationally. The actual frequency of APS in the general population is unknown. The annual incidence of APS has been estimated at approximately 5 cases per 100,000 persons, and the prevalence is approximately 40-50 cases per 100,000 persons. [13, 14]

One to 5% of healthy individuals have aPL antibodies. aPL antibodies tend to be found more frequently in elderly persons; thus, positive titer results should be interpreted with caution in this population. aPL antibodies are found in approximately 30-40% of patients with SLE, but only about 10% have APS. [15] Approximately half of APS cases are not associated with another rheumatic disease. In a study of 100 patients with confirmed venous thrombosis and no history of SLE, anticardiolipin antibodies were found in 24% and lupus anticoagulant in 4%.

The approximate frequency of positive aPL titers in patients with other disorders is as follows [16] :

  • Stroke: 13%
  • Myocardial infarction: 11%
  • Deep venous thrombosis: 9.5%
  • Pregnancy morbidity: 6%

 Race-, Sex-, and Age-related Demographics

No defined racial predominance for primary APS has been documented, although SLE is more common in African- American and Hispanic populations.

A female predominance has been documented, particularly for secondary APS. This parallels the association of APS with SLE and other connective-tissue diseases, which also have a female predominance.

APS is more common in young to middle-aged adults; however, it also manifests in children and elderly people. Disease onset has been reported in children as young as 8 months. In an international registry of pediatric APS cases, patients without associated rheumatic disease were younger and had a higher frequency of arterial thrombotic events, whereas patients with associated rheumatic disease were older and had a higher frequency of venous thrombotic events associated with hematologic and skin manifestations. [17]


APS may contribute to an increased frequency of stroke or MI, especially in younger individuals. Strokes may develop secondary to in situ thrombosis or embolization that originates from the valvular lesions of Libman-Sacks (sterile) endocarditis, which may be seen in patients with APS. Cardiac valvular disease may be severe enough to require valve replacement. Recurrent pulmonary emboli or thrombosis can lead to life-threatening pulmonary hypertension.

Catastrophic APS (CAPS) is a rare, serious, and often fatal manifestation characterized by multiorgan infarctions over a period of days to weeks. Mortality rates of 50% have been reported; however, with triple therapy (anticoagulation, corticosteroids, plasma exchange and/or intravenous immunoglobulin) data from an international registry showed a mortality rate of 28.6%. [18]

Late spontaneous fetal loss (second or third trimester) is common; however, it can occur at any time during pregnancy. Recurrent early fetal loss (< 10 weeks’ gestation) is also possible.