Clinical Information

Clinical Indications

• Evaluation of patients with suspected antiphospholipid syndrome (APS)
• First-line laboratory assessment of APS in conjunction with:
• Lupus anticoagulant (LAC)
• Anticardiolipin antibodies (aCL) IgG and IgM
• Estimating risk of thrombosis and/or pregnancy-related morbidity, particularly in patients with systemic lupus erythematosus (SLE)

Clinical Background

Antiphospholipid syndrome (APS) is a systemic autoimmune disorder characterized by:

• Arterial or venous thrombosis
• Specific pregnancy complications (e.g., recurrent pregnancy loss, preeclampsia, placental insufficiency)

Diagnosis of APS requires both clinical findings and persistent positivity of one or more criteria antiphospholipid (aPL) antibodies.

Role in APS Classification

Based on the 2006 revised Sapporo criteria and the 2023 ACR/EULAR APS classification criteria, laboratory evaluation for APS includes:

• Lupus anticoagulant (LAC)
• Anticardiolipin (aCL) IgG and IgM
• Anti–beta-2 glycoprotein I (anti-β2GPI) IgG and IgM

The 2023 ACR/EULAR criteria are primarily intended for research classification and may not always apply directly to routine clinical decision-making.

Anti-β2GPI antibodies are detected using enzyme-linked immunosorbent assay (ELISA). Unlike lupus anticoagulant testing, which is functional, anti-β2GPI testing provides:

• Semi-quantitative results
• Isotype-specific information (IgG vs IgM)
• Risk stratification value

Clinical Considerations

• APS diagnosis requires persistent positivity, defined as detection on two or more occasions at least 12 weeks apart.
• Antiphospholipid antibodies may occur transiently during infections.
• aPL antibodies may be present in autoimmune diseases, especially SLE.
• Risk is higher in patients with double or triple antibody positivity (LAC + aCL + anti-β2GPI).

Testing for anti-β2GPI antibodies is not affected by anticoagulant therapy.

Reference Intervals

Beta-2 Glycoprotein I (β2GPI) IgG

• <15.0 SGU — Negative
• 15.0–39.9 SGU — Weakly positive
• 40.0–79.9 SGU — Positive
• ≥80.0 SGU — Strongly positive

Results reported in Standard IgG Units (SGU).

Beta-2 Glycoprotein I (β2GPI) IgM

• <15.0 SMU — Negative
• 15.0–39.9 SMU — Weakly positive
• 40.0–79.9 SMU — Positive
• ≥80.0 SMU — Strongly positive

Results reported in Standard IgM Units (SMU).

Reference values apply to all ages.

Result Interpretation

• Moderate-to-strong positive anti-β2GPI IgG or IgM results, in the appropriate clinical context, support the diagnosis of APS.
• Among isotypes, IgG anti-β2GPI antibodies carry higher diagnostic and thrombotic risk significance than IgM. Isolated low-level IgM positivity carries lower risk and should be interpreted cautiously.
• A negative result does not exclude APS. If clinical suspicion remains high, lupus anticoagulant and anticardiolipin testing should be performed.

Results must be interpreted in conjunction with clinical findings and other laboratory tests. Treatment decisions should not be based on antibody results alone.

Cautions and Limitations

• Antiphospholipid antibodies may occur transiently during infections. Immunoassays may not fully distinguish APS-specific antibodies from those associated with infections.
• Results from different manufacturers are not interchangeable. Comparative studies indicate inter-assay variability; therefore, serial testing should ideally be performed using the same method.
• The magnitude of antibody level cannot be directly correlated with endpoint titers.
• Assay performance characteristics have been established for serum only.