Antiphospholipid Syndrome - APS

Last Literature Review: February 2024 Last Update:

Antiphospholipid syndrome (APS), also known as antiphospholipid antibody syndrome, is an autoimmune disorder in which autoantibodies are directed against phospholipid-protein complexes. APS is characterized by thromboses (arterial, venous, or small vessel) and/or pregnancy complications and persistently positive tests for antiphospholipid-protein (aPL) antibodies.  Cytopenias, other hematologic disorders, and neurologic, dermatologic, or cardiopulmonary abnormalities may also be seen in patients with APS. ,  An uncommon acute form of the syndrome, catastrophic APS, results in extensive microvascular thrombosis and multiorgan failure. Those at increased risk for APS include patients with systemic lupus erythematosus (SLE), infections, malignancy, and liver or vascular disease. Transient aPL antibodies may occur in association with infections, certain medications (eg, procainamide and chlorpromazine), and malignancy. 

Quick Answers for Clinicians

Are there any formal diagnostic criteria for antiphospholipid syndrome?

Currently, no formal diagnostic criteria exist for antiphospholipid syndrome (APS). However, the revised Sapporo (Sydney) classification system for APS,  which comprises clinical and laboratory criteria intended for clinical research, has been used in a clinical setting to inform diagnosis. Refer to the Classification Criteria section for specific details.

Although a new classification system for APS was released in 2023 by the American College of Rheumatology (ACR) and the European Alliance of Associations for Rheumatology (EULAR),  experts disagree about the use of these criteria in clinical practice. Notably, the International Society on Thrombosis and Haemostasis (ISTH) has expressed hesitancy about the routine clinical use of criteria that are primarily designed to standardize research efforts.  In addition, experts in laboratory medicine have pointed out that the ACR-EULAR’s recommendation to use enzyme-linked immunosorbent assay (ELISA)-only testing for antiphospholipid-protein (aPL) antibodies may present a challenge due to the prevalence and advantages of automated testing, as well as a lack of harmonization between the aPL ELISAs currently available. 

Which lab tests are most useful for diagnosing antiphospholipid syndrome?

Three test groups, used together, are recommended for antiphospholipid syndrome (APS) diagnosis. They include lupus anticoagulant (LA), anticardiolipin (aCL) antibodies (immunoglobulin G [IgG] and IgM), and anti-beta-2 glycoprotein 1 (anti-β2GP1) antibodies (IgG and/or IgM). If one or more of these tests are positive, the test(s) should be repeated at least 12 weeks later to confirm persistent positivity. , ,  If the tests are negative but strong suspicion for APS remains, “noncriteria” tests may be indicated. Refer to the Noncriteria Tests section for additional information.

What factors are important in antiphospholipid syndrome testing?

Testing during acute thrombotic episodes is not recommended, as the presence of acute phase reactants (eg, factor VIII or C-reactive protein) can lead to false-positive and false-negative lupus anticoagulant (LA) test results. The presence of anticoagulant therapies can also produce spurious LA test results; therefore, whenever possible, testing should be performed when anticoagulants are not present. In some cases, testing in the absence of anticoagulants may not be feasible, in which case testing that includes anticoagulant measurement and neutralization (eg, direct oral anticoagulant adsorption) may be useful. 

Because antiphospholipid-protein (aPL) antibodies can occur transiently, persistent positivity on consecutive testing occasions is required for classification as antiphospholipid syndrome (APS).  In addition, testing for LA, anticardiolipin (aCL) antibodies, and anti-beta-2 glycoprotein 1 (anti-β2GP1) antibodies reduces the risk of false-negative findings and helps in assessing risk for complications such as thrombotic events. 

Indications for Testing

Laboratory testing for APS is appropriate in individuals who have an increased likelihood of the disorder, including those with , , , :

  • Arterial thrombosis, unprovoked venous thrombosis, or evidence of brain ischemia before 50 years of age
  • Unexplained recurrent thrombosis
  • Thrombosis at an unusual site
  • Extensive microvascular thrombi
  • Unexplained pregnancy loss, either prefetal or fetal (at or after 10 weeks of gestation)
  • Severe preeclampsia/eclampsia; hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome; or severe placental insufficiency
  • A diagnosis of SLE

Testing may also be considered in other specific clinical scenarios; for additional information, refer to the International Society on Thrombosis and Haemostasis guidelines  and the classification criteria listed in the following section.

Classification Criteria

Although no formal diagnostic criteria exist for APS, the following classification system (which includes clinical and laboratory criteria intended for clinical research studies) can be used to inform diagnosis.

Revised Sapporo (Sydney) Classification Criteria for APSa
Clinical Criteria
Vascular thrombosis≥1 clinical episodes of arterial, venous, or small-vessel thrombosis in any tissue or organ, validated by imaging studies or histopathology
Pregnancy morbidity

≥1 unexplained deaths of a morphologically normal fetus at or after 10 wks of gestation

≥1 premature births of a morphologically normal neonate before 34 wks of gestation due to preeclampsia, eclampsia, or placental insufficiency

≥3 unexplained, consecutive, spontaneous abortions before 10 wks of gestation

Laboratory Criteria
Positive for LA or aCL or anti-β2GP1 antibodies (according to specified criteria) on ≥2 occasions at least 12 wks apart

LA: detected in plasma according to ISTH guidelines

aCL antibodies: IgG and/or IgM isotype present in a medium or high titer (>40 GPL or MPL units or >99th percentile), as measured by standardized ELISA

Anti-β2GP1 antibodies: IgG and/or IgM isotype in high titer (>99th percentile), measured by standardized ELISA

aAt least 1 clinical and 1 laboratory criterion must be met within a 5-yr period.

aCL, anticardiolipin; anti-β2GP1, anti-beta-2 glycoprotein 1; ELISA, enzyme-linked immunosorbent assay; GPL, immunoglobulin G (IgG) phospholipid antibody; ISTH, International Society on Thrombosis and Haemostasis; LA, lupus anticoagulant; MPL, IgM phospholipid antibody

Sources: Miyakis, 2006 ; Bertolaccini, 2014 ; Devreese, 2018 

Laboratory Testing


First-Line or Criteria Tests

Recommended first-line testing for possible APS includes LA clot-based assays, tests for aCL IgG and IgM antibodies, and tests for anti-β2GP1 IgG and IgM antibodies, , ,  which are available separately or as a panel. The combination of these three tests reduces the risk of false-negative findings and is important for estimating disease risk.  Positivity for all three (LA as well as aCL and anti-β2GP1 antibodies) is strongly associated with thromboembolism and pregnancy-related morbidity.  Triple positivity also indicates a high risk of thrombotic recurrence in patients with APS. 

If positive, laboratory tests should be repeated at least 12 weeks later to confirm persistent positivity. 

Lupus Anticoagulant

At least two phospholipid-dependent clotting assays should be performed in parallel to identify LA. ,  Assays should include appropriate reflexive steps (eg, screening, mixing, confirmation) and should be based on different principles (eg, activated partial thromboplastin time [aPTT] and dilute Russell viper venom time [dRVVT]).  If either clotting assay produces positive results, LA should be considered present. 

Because anticoagulant therapy can produce false-positive and false-negative LA test results, when possible, specimens should be collected from patients who are not taking anticoagulants.  In patients able to tolerate a temporary pause in direct oral anticoagulant (DOAC) therapy, LA testing can be considered at least 48 hours postdose with concurrent measurement of DOAC concentration. DOAC or heparin neutralization may be used to minimize test interference when these anticoagulants are present. , 

Positivity for LA alone, apart from the other aPLs, has a strong association with thrombotic events and adverse outcomes in pregnancy. , ,  However, LA testing in pregnant individuals may produce false-positive or false-negative results. If LA testing is performed during pregnancy, testing should be repeated postdelivery to confirm aPL status. Testing 3 months postdelivery may be reasonable, but an exact time frame has not been fully established. 

Anticardiolipin and Anti-Beta-2 Glycoprotein 1 Antibodies

A strong correlation has been observed between aCL and anti-β2GP1 antibody concentrations, but testing for both antibody types in conjunction with LA is still recommended. Detecting aCL and anti-β2GP1 antibodies of the same isotype (IgG or IgM) supports the likelihood of APS. Measurement of both IgG and IgM is advised. 

Noncriteria Tests

In recent decades, “noncriteria” aPL tests have been investigated for their potential value in APS diagnosis or risk assessment. Although their role is still unclear and routine use is not recommended, when criteria tests are negative but a strong suspicion for APS remains, noncriteria tests can be considered. These include tests for phosphatidylserine/prothrombin antibodies (IgG and IgM), phosphatidylserine antibodies (IgG and IgM), prothrombin antibody (IgG), anti-β2GP1 antibody (IgA), and aCL antibody (IgA).

ARUP Laboratory Tests

For additional APS testing from ARUP Laboratories, refer to the Laboratory Test Directory.

First-Line or Criteria Tests
Noncriteria Tests


Medical Experts



Karen A. Moser, MD
Associate Professor of Pathology (Clinical), University of Utah
Medical Director, Hemostasis/Thrombosis, ARUP Laboratories


Kristi J. Smock, MD
Professor of Pathology (Clinical), University of Utah
Chief Medical Director, ARUP Institute for Clinical and Experimental Pathology
Medical Director, Hemostasis/Thrombosis, ARUP Laboratories