Antiphospholipid Syndrome: Blood Clots, Miscarriages, and Why This Autoimmune Condition Is Often Found Late

Research-informed explainer — last updated April 12, 2026

Antiphospholipid syndrome (APS) is an autoimmune condition in which the immune system produces antibodies that attack normal proteins in the blood, causing it to clot too easily — yet most patients do not receive the diagnosis until after a second or third thrombotic event or pregnancy loss. Understanding the antibodies involved, the classification criteria, and the long-term anticoagulation strategy can be life-saving for patients and their families.

This article draws on research by Jason Knight, MD, Professor at the University of Michigan (Michigan Medicine), who contributed to the 2023 ACR/EULAR APS Classification Criteria (390 and 379 citations) and led the discovery that neutrophil extracellular traps (NETs) stimulated by antiphospholipid antibodies are a key thrombosis mechanism (372 citations); Kenneth Warrington, MD, Chair of Rheumatology at Mayo Clinic in Rochester, whose population-based epidemiology of APS (277 citations) defines incidence and prevalence; Rachel Kaiser, MD MPH, whose work on risk and protective factors for thrombosis in SLE with APS antibodies (240 citations) and contraceptive counseling in autoimmune disease (127 citations) directly addresses the obstetric and reproductive health questions most patients ask; Joan Bathon, MD, Chief Emerita of the Division of Rheumatology at NewYork-Presbyterian/Columbia University Irving Medical Center, who provides clinical expertise on long-term management outcomes in autoimmune thrombotic disease; and Arezou Khosroshahi, MD, Assistant Professor of Medicine at Emory University School of Medicine, whose autoimmune disease expertise informs the overlap differential.

What is antiphospholipid syndrome?

APS is defined by two components that must both be present: (1) at least one clinical manifestation — arterial thrombosis, venous thrombosis, or obstetric complications — and (2) persistent antiphospholipid antibodies confirmed on at least two tests 12 weeks apart. The condition occurs in approximately 2 per 100,000 people annually, with a prevalence of about 50 per 100,000 in the general population. It disproportionately affects women of childbearing age. About 30–40% of cases occur in the context of systemic lupus erythematosus (SLE); the remainder are "primary" APS without another underlying autoimmune disease.

The three antibodies that define the condition

Three main antibody types are tested, and their combination matters greatly for prognosis:

• Lupus anticoagulant (LA): paradoxically named — it prolongs clotting tests in the lab but causes clotting in patients; the strongest predictor of thrombosis
• Anticardiolipin antibodies (aCL): IgG and IgM isotypes; higher titers correlate with higher risk
• Anti-beta-2 glycoprotein I antibodies (anti-β2GPI): the most specific antibody for the diagnosis

Triple positivity — positive for all three — is associated with the highest risk of thrombosis and pregnancy complications. Low positive results for a single antibody, particularly IgM aCL, carry much lower risk and may represent transient positivity related to infection rather than true APS.

The 2023 classification update: why it matters

The 2023 ACR/EULAR classification criteria (390 and 379 citations), to which Dr. Knight contributed, replaced older criteria from 2006. The update introduced a domain-based scoring system that weights antibody types and clinical manifestations differently rather than treating all positive tests as equivalent. High-risk antibody profiles (triple positivity, high-titer LA) score more heavily than low-risk profiles. This change aims to reduce misclassification of patients with transient, low-level antibody positivity as having definite APS.

How antiphospholipid antibodies cause clots: the NET connection

Dr. Knight's laboratory work, published in Arthritis & Rheumatology (372 citations), demonstrated that antiphospholipid antibodies stimulate neutrophils to release neutrophil extracellular traps (NETs) — web-like structures of DNA and proteins that capture platelets and promote fibrin formation. This mechanism explains why APS patients form clots in both veins and arteries and why the thrombotic risk is systemic rather than localized. It also opens potential future targets for therapy beyond anticoagulation.

Clinical manifestations: clots, pregnancy loss, and beyond

Thrombosis: DVT and pulmonary embolism are most common. Arterial events — stroke, transient ischemic attack, and myocardial infarction — occur more often in APS than in thrombophilia from other causes. Recurrence rates without anticoagulation are high: approximately 20–30% per year after an initial event.

Obstetric APS: includes recurrent early miscarriage (before 10 weeks), late pregnancy loss (after 10 weeks), and preeclampsia or placental insufficiency with preterm birth. These complications are the most common presentation in women of reproductive age, often occurring before any thrombotic event.

Catastrophic APS (CAPS): a rare but life-threatening variant in which multiple organ systems clot simultaneously within days. Mortality is high (30–50%) despite treatment.

The SLE overlap and contraception

Dr. Kaiser's work on thrombosis risk in SLE (240 citations) identified that lupus anticoagulant is the single strongest risk factor for thrombosis in SLE-associated APS. Her contraceptive counseling study (127 citations) documented that many women with lupus and APS are not receiving appropriate guidance — including the avoidance of estrogen-containing contraceptives, which substantially raise clotting risk. Non-hormonal contraception or progestin-only methods are the standard recommendation for women with APS.

Treatment: anticoagulation is the cornerstone

For thrombotic APS, indefinite anticoagulation with warfarin is the established standard. Target INR is 2–3 for venous thrombosis and 3–4 for arterial events or recurrence on standard anticoagulation. Direct oral anticoagulants (DOACs) — rivaroxaban, apixaban — have been studied in APS trials and have shown higher rates of arterial thrombosis recurrence in triple-positive patients; warfarin remains preferred for high-risk profiles.

For obstetric APS without prior thrombosis, the standard of care is low-dose aspirin combined with low molecular weight heparin throughout pregnancy.

Questions to ask your doctor

• Which of the three antiphospholipid antibodies am I positive for, and were they confirmed on a second test 12 weeks later?
• Is my antibody profile high-risk (triple positive) or lower-risk (single positive low titer)?
• Should I be on anticoagulation indefinitely, and what INR target is appropriate for my case?
• Do I have SLE or another autoimmune condition that increases my risk further?
• If I want to become pregnant, what medications and monitoring plan would be used during my pregnancy?
• Are estrogen-containing contraceptives safe for me to use?

The bottom line

Antiphospholipid syndrome is an autoimmune cause of blood clots and pregnancy loss that is often diagnosed late because individual events are attributed to other causes. The 2023 ACR/EULAR criteria use a more nuanced antibody-weighted scoring approach, and the treatment backbone — long-term anticoagulation with warfarin for most thrombotic presentations — has strong evidence supporting it. Women with APS planning pregnancy or considering contraception need specialist guidance to manage reproductive health safely.