Diagnostic Criteria for Antiphospholipid Syndrome

Clinical and Laboratory Criteria

• The American College of Rheumatology recommends that definite Antiphospholipid Syndrome (APS) requires both at least one clinical criterion (thrombosis or pregnancy morbidity) and at least one laboratory criterion (lupus anticoagulant, anticardiolipin antibodies, or anti-β2-glycoprotein I antibodies), with positive laboratory tests confirmed on two separate occasions at least 12 weeks apart 1
• The American College of Rheumatology suggests that three or more consecutive spontaneous abortions before 10 weeks of gestation, with maternal anatomic/hormonal abnormalities and paternal/maternal chromosomal causes excluded, is a clinical criterion for APS 2
• The American College of Rheumatology recommends that one or more unexplained fetal deaths at or after 10 weeks of gestation, with normal fetal morphology documented by ultrasound or direct examination, is a clinical criterion for APS 2
• The American College of Rheumatology suggests that one or more premature births before 34 weeks due to eclampsia, severe preeclampsia, or recognized features of placental insufficiency (intrauterine growth restriction or fetal distress) is a clinical criterion for APS 2

Laboratory Testing

• The International Society on Thrombosis and Haemostasis recommends that lupus anticoagulant (LAC) should be detected in plasma using phospholipid-dependent clotting assays, and requires a combination of two phospholipid-dependent clotting assays (no single test has sufficient sensitivity and specificity) 1, 3
• The International Society on Thrombosis and Haemostasis suggests that β2-glycoprotein I-dependent anticardiolipin antibodies (aCL) should be present at moderate-to-high titer (>40 units or >99th percentile) and measured by solid-phase assays (ELISA or automated systems) 1, 2, 3, 4
• The International Society on Thrombosis and Haemostasis recommends that anti-β2-glycoprotein I antibodies (aβ2GPI) should be present at moderate-to-high titer (>40 units or >99th percentile) and measured by solid-phase assays (ELISA or automated systems) 1, 2, 3, 4

Risk Stratification

• The American College of Rheumatology suggests that triple positivity (LAC + aCL + aβ2GPI of the same isotype) indicates the highest risk of recurrent pregnancy complications and thrombotic events 5, 6
• The American College of Rheumatology recommends that patients with lower-titer aCL and/or aβ2GPI (<40 units) who do not meet laboratory classification criteria may still have some degree of risk that is difficult to quantify 2

Testing Limitations

• The International Society on Thrombosis and Haemostasis recommends that LAC testing in patients on anticoagulation therapy may cause erroneous results 3
• The International Society on Thrombosis and Haemostasis suggests that other antiphospholipid antibody tests beyond the three criteria tests are not recommended yet for diagnostic purposes 3, 4

When to Test for APS

• The American College of Rheumatology recommends that testing for aPL should be performed in patients with SLE or SLE-like disease, patients with suggestive histories (unexplained thrombosis, recurrent pregnancy loss, thrombocytopenia), and patients with suggestive physical findings (livedo reticularis, cardiac valve abnormalities) 2, 6

Antiphospholipid Antibody Testing: Evidence‑Based Recommendations

Laboratory Confirmation Requirements

• Confirm all positive antiphospholipid antibody results (lupus anticoagulant, anticardiolipin IgG/IgM, anti‑β2‑glycoprotein I IgG/IgM) with repeat testing performed ≥ 12 weeks later, because transient positivity is common with infections and medications. 7

Risk Stratification Based on Antibody Profiles

• Triple positivity (simultaneous positivity for lupus anticoagulant, anticardiolipin, and anti‑β2‑glycoprotein I of the same isotype) confers the highest risk for both recurrent pregnancy complications and thrombotic events. 7
• Triple‑positive patients exhibit the strongest association with subsequent thrombosis compared with any other antibody pattern. 7
• Double‑positive patients (negative lupus anticoagulant but positive anticardiolipin and anti‑β2‑glycoprotein I of the same isotype) meet clinical antiphospholipid‑syndrome criteria in ≈ 86 % of cases. 7
• Isolated lupus anticoagulant positivity carries a high predictive value for thrombosis and is given significant weight in the classification criteria. 7
• Isolated anticardiolipin or anti‑β2‑glycoprotein I positivity alone shows inconsistent or weak association with thrombotic outcomes. 7
• Lupus anticoagulant is the strongest laboratory risk factor for pregnancy morbidity; when it co‑exists with anti‑β2‑glycoprotein I positivity, the predictive value for adverse obstetric outcomes is highest. 7

Testing Timing Pitfalls

• Never diagnose APS on a single positive test; repeat testing after a minimum interval of 12 weeks is mandatory to exclude transient positivity. 7
• Performing antibody testing during an acute thrombotic event may yield falsely low anti‑β2‑glycoprotein I titers because antibodies can be sequestered at the clot site; retesting after the event is recommended. 7
• During pregnancy, antiphospholipid‑antibody levels can fluctuate; ≈ 25 % of lupus‑anticoagulant‑positive patients become negative in the second or third trimester due to rising Factor VIII, so initial diagnosis should not rely on testing performed in pregnancy. 7
• If initial testing occurs during pregnancy, an acute thrombotic episode, or shortly after delivery, repeat testing at a later, stable time point is required to confirm persistent positivity. 7

Impact of Anticoagulants on Antiphospholipid Antibody Testing

Assay Interference by Direct Oral Anticoagulants (DOACs)

• Direct oral anticoagulants, including apixaban, can produce unreliable results on lupus‑anticoagulant (LA) functional coagulation assays, leading to false‑positive or false‑negative interpretations. 8
• Vitamin K antagonists such as warfarin similarly interfere with LA functional testing, but they do not affect solid‑phase measurements of anticardiolipin or anti‑β2‑glycoprotein I antibodies. 8

Reliability of Solid‑Phase Antibody Assays During Anticoagulant Therapy

• Anticardiolipin IgM and anti‑β2‑glycoprotein I IgM antibodies are detected by solid‑phase ELISA assays, which identify true autoimmune antibodies and are not altered by the presence of apixaban or other DOACs. [8][9]

Common Causes of Transient Antiphospholipid Antibody Positivity

• Acute infections frequently induce temporary antiphospholipid antibody positivity; therefore, confirmation testing at least 12 weeks apart is required before diagnosing antiphospholipid syndrome. 9

Testing Recommendations for Suspected Antiphospholipid Syndrome (APS)

• A single positive antiphospholipid antibody test is insufficient for APS diagnosis; repeat testing after a minimum interval of 12 weeks is mandatory to exclude transient positivity. 9
• When a patient on apixaban shows a positive anticardiolipin IgM or anti‑β2‑glycoprotein I IgM result, the medication is not the cause—investigate alternative etiologies such as infection, underlying autoimmune disease, or pregnancy. 8
• Laboratory interference concerns apply only to lupus‑anticoagulant functional assays; ELISA‑based anticardiolipin and anti‑β2‑glycoprotein I testing remain reliable in the setting of DOAC therapy. 8