What is Antiphospholipid Antibody Syndrome (APAS)?

What is Antiphospholipid Antibody Syndrome (APAS/APS)?

Antiphospholipid syndrome (APS) is a thrombo-inflammatory autoimmune disease driven by antiphospholipid antibodies (aPL) that cause thrombosis (blood clots in veins and/or arteries), pregnancy complications, and other inflammatory manifestations. 1

Core Pathophysiology

APS occurs when autoantibodies recognize and bind to phospholipid surfaces and phospholipid-binding proteins, particularly beta-2 glycoprotein I (β2GPI). This antibody-antigen interaction triggers:

• Endothelial cell damage and dysfunction
• Platelet activation
• Activation of inflammatory signaling pathways
• Interference with the coagulation cascade

These mechanisms collectively create a hypercoagulable state that predisposes patients to thrombotic events 1, 2.

Clinical Manifestations

Thrombotic Events

APS can affect vessels of any size:

• Venous thrombosis (most common): deep vein thrombosis, pulmonary embolism
• Arterial thrombosis: stroke, myocardial infarction
• Small vessel thrombosis: can lead to multi-organ involvement

Pregnancy Morbidity

• Recurrent fetal losses (typically ≥3 consecutive miscarriages before 10 weeks)
• One or more unexplained deaths of morphologically normal fetuses ≥10 weeks
• Premature birth before 34 weeks due to eclampsia, preeclampsia, or placental insufficiency

Non-Criteria Manifestations

Additional features that add significant morbidity:

• Thrombocytopenia
• Livedo reticularis (net-like skin discoloration)
• Cardiac valve disease
• Nephropathy
• Cognitive dysfunction
• Skin ulcers 3

Classification

Primary APS: Occurs as an isolated condition without other autoimmune diseases

Secondary APS: Associated with underlying autoimmune diseases, most commonly systemic lupus erythematosus (SLE) 4, 5

Catastrophic APS (CAPS): A rare, life-threatening variant characterized by rapid-onset widespread microthrombosis leading to multi-organ failure, often triggered by infections, surgery, or cessation of anticoagulation 6

Laboratory Diagnosis

The cornerstone laboratory tests remain lupus anticoagulant (LA), anticardiolipin antibodies (aCL), and anti-β2 glycoprotein I antibodies (aβ2GPI). 1

Current Testing Standards (2023 ACR/EULAR Criteria)

Per the most recent 2025 ISTH guidance 1:

• aCL and aβ2GPI: Measured by ELISA
• Moderate titer threshold: ≥40 Units
• High titer threshold: ≥80 Units
• Isotypes tested: IgG and IgM (both should be measured)
• Lupus anticoagulant: Requires two parallel tests (dilute Russell's viper venom time [dRVVT] and LA-sensitive activated partial thromboplastin time [APTT])

Critical Testing Requirements

Persistent positivity is mandatory: Antibodies must be present on two or more occasions at least 12 weeks apart to confirm diagnosis and exclude transient positivity 7. This repeat testing ensures reliability given poor standardization and potential interferences.

Risk Stratification by Antibody Profile

The antibody profile significantly impacts thrombotic risk 7:

• Triple-positive (LA + aCL + aβ2GPI of same isotype): Highest risk for thrombosis and recurrence
• Double-positive (typically LA negative): Lower risk than triple-positive
• Single-positive (LA, aCL, or aβ2GPI alone): Lowest risk, though LA positivity alone remains a strong predictor, especially for arterial thrombosis and adverse pregnancy outcomes

Important Clinical Pitfalls

Classification vs. Clinical Diagnosis

A critical distinction exists between classification criteria (used for research) and clinical diagnosis (used for patient care). 1

• Classification criteria are strict and meant for homogeneous research populations
• Clinical diagnosis in daily practice is broader to optimize individual patient management
• Patients can have APS without fulfilling classification criteria—inappropriate use of classification criteria leads to underdiagnosis

Seronegative APS

Some patients present with typical APS clinical features but have negative standard antibody tests, possibly due to:

• Non-criteria antibodies (e.g., anti-phosphatidylserine/prothrombin antibodies)
• Transient immunosuppression
• Testing limitations 2

Prevalence vs. Incidence

Approximately 5% of the general population has circulating aPL antibodies, but APS incidence is only 5 per 100,000 people because diagnosis requires both clinical manifestations AND laboratory findings to be simultaneously present 5.

Treatment Principles

Long-term anticoagulation with vitamin K antagonists (VKAs/warfarin) remains the mainstay of treatment for most APS patients. 4

• Direct oral anticoagulants (DOACs) are generally not recommended for APS, particularly in triple-positive or arterial thrombosis cases
• Heparin and aspirin are used in pregnancy-related APS
• Catastrophic APS requires urgent intensive therapy: anticoagulation + high-dose corticosteroids + plasma exchange + intravenous immunoglobulin 6

The autoimmune nature of APS notwithstanding, treatment remains primarily based on anticoagulation and antiplatelet therapy rather than immunosuppression, as standard immunosuppressive therapies and B-cell depletion have proven ineffective 3, 8.