Antiphospholipid Syndrome: Diagnosis & Anticoagulation
Clots in odd places, lost pregnancies, and why warfarin still wins
Overview and pathophysiology
Definition. Antiphospholipid syndrome (APS) is an acquired autoimmune thrombophilia defined by thrombosis or pregnancy morbidity plus persistent antiphospholipid antibodies (aPL). The antibodies target phospholipid-binding plasma proteins—above all β2-glycoprotein I—and activate endothelial cells, platelets, and complement. The result is venous, arterial, and small-vessel thrombosis, and placental insufficiency.
Primary vs secondary. APS occurs alone (primary) or with another autoimmune disease, chiefly SLE; about 30–40% of lupus patients carry aPL. Having the antibodies is not the same as having the syndrome—clinical events are required.
When to suspect APS
Thrombotic presentations. Test for aPL after venous thromboembolism that is unprovoked in a young patient, thrombosis at unusual sites (cerebral venous sinuses, hepatic or portal veins, mesenteric veins), stroke or MI before about age 50 without vascular risk factors, or recurrent clots despite anticoagulation. A DVT after a fracture, or a stroke in an older patient with atrial fibrillation, has an obvious explanation and does not by itself warrant testing.
Obstetric presentations. Classic (2006 revised Sapporo) criteria are ≥3 consecutive unexplained pregnancy losses before 10 weeks, ≥1 unexplained death of a morphologically normal fetus at ≥10 weeks, or delivery before 34 weeks because of severe preeclampsia, eclampsia, or placental insufficiency. The 2023 ACR/EULAR classification criteria, used mainly for research, instead weight clinical and laboratory domains (for example, early losses and isolated IgM antibodies score low), but the same tests and 12-week confirmation underpin both.
Supportive clues.
- Livedo reticularis; livedo plus strokes in a young adult is Sneddon syndrome, often aPL-positive.
- Mild to moderate thrombocytopenia.
- Libman–Sacks (nonbacterial verrucous) endocarditis, usually of the mitral valve, with embolic stroke risk.
- APS nephropathy, chorea, and cognitive deficits.
- A prolonged aPTT that fails to correct with a 1:1 mix of normal plasma—an inhibitor pattern—in a patient who clots rather than bleeds. Factor deficiencies (factor VIII, vitamin K deficiency) correct on mixing; an acquired factor VIII inhibitor also fails to correct but causes bleeding.
- A false-positive VDRL/RPR: anticardiolipin reacts with the cardiolipin antigen in nontreponemal tests, while treponemal tests are negative. Test for aPL rather than treating syphilis.
"Lupus anticoagulant" is a double misnomer: most patients do not have lupus, and the antibody prolongs clotting times in the test tube but is prothrombotic in the body.
Laboratory diagnosis
| Test | Method and positive result | Pitfall |
|---|---|---|
| Lupus anticoagulant | Clot-based: dRVVT and LA-sensitive aPTT, with correction by excess phospholipid confirming dependence | Unreliable on warfarin, heparin, or DOACs |
| Anticardiolipin IgG/IgM | Immunoassay; moderate–high titer (>40 units or >99th percentile) | Low titers are common and low risk |
| Anti-β2-glycoprotein I IgG/IgM | Immunoassay; >99th percentile | Unaffected by anticoagulants |
- Clinical eventThrombosis or obstetric morbidity prompts testing
- Initial panelLupus anticoagulant, anticardiolipin, and anti-β2GPI—ideally drawn before anticoagulation starts
- Repeat at ≥12 weeksTransient, often low-titer antibodies follow infections and some drugs (e.g., chlorpromazine, procainamide)
- Persistently positiveAPS confirmed; stratify risk and plan long-term therapy
Testing on anticoagulants. Warfarin, heparin, and DOACs distort clot-based LA assays, producing false positives and negatives; warfarin also lowers the vitamin K–dependent proteins C and S. Anticardiolipin and anti-β2GPI immunoassays remain reliable and can be sent at any time.
Risk profile. Triple positivity (LA + anticardiolipin + anti-β2GPI) carries the highest thrombotic risk, followed by LA alone, persistently high titers, and coexisting SLE. An isolated low-titer anticardiolipin carries little risk.
Long-term anticoagulation
Thrombotic APS. Start heparin and overlap with warfarin for ≥5 days and until the INR is ≥2 for 24 hours, then continue a vitamin K antagonist at INR 2–3 indefinitely because recurrence after stopping is common. After arterial events, some patients receive added low-dose aspirin or a higher INR target.
Avoid DOACs in APS, especially with triple positivity or arterial thrombosis. In the TRAPS trial, rivaroxaban produced more arterial events (stroke, MI) than warfarin in triple-positive patients.
| Scenario | Therapy | Notes |
|---|---|---|
| Venous thrombosis | Warfarin, INR 2–3, indefinitely | Heparin overlap at the start |
| Arterial thrombosis | Warfarin ± low-dose aspirin or higher INR target | No DOACs |
| Obstetric APS, no prior clot | Prophylactic LMWH + low-dose aspirin | Through ~6 weeks postpartum |
| Pregnancy with prior thrombosis | Therapeutic LMWH + low-dose aspirin | Switch off warfarin before 6 weeks |
| Asymptomatic high-risk profile | Low-dose aspirin | LMWH prophylaxis in high-risk settings |
| Catastrophic APS | Heparin + high-dose glucocorticoids + plasma exchange and/or IVIG | Treat the trigger |
Pregnancy and asymptomatic carriers
Obstetric APS. Women with obstetric APS and no thrombosis receive low-dose aspirin plus prophylactic-dose LMWH throughout pregnancy and for about 6 weeks postpartum; glucocorticoids do not improve outcomes. Women with prior thrombosis need therapeutic-dose LMWH plus low-dose aspirin.
Warfarin and DOACs in pregnancy. Warfarin is teratogenic—exposure at 6–12 weeks causes nasal hypoplasia and stippled epiphyses—so women on warfarin switch to LMWH before 6 weeks' gestation. DOACs are avoided. After delivery, women with thrombotic APS resume warfarin; both warfarin and LMWH are compatible with breastfeeding.
Antibodies without events. Carriers with a high-risk profile (triple positive, LA, persistently high titers) take low-dose aspirin—not full anticoagulation—and receive LMWH prophylaxis during surgery, immobility, or the postpartum period. aPL-positive women should avoid estrogen-containing contraceptives and use an IUD (copper preferred; levonorgestrel acceptable) or a progestin-only pill; avoid depot medroxyprogesterone acetate, which raises thrombosis risk.
Catastrophic APS
Recognition. Catastrophic APS is thrombosis of ≥3 organs, systems, or tissues developing in less than a week, with small-vessel occlusion on biopsy and aPL present. Kidneys, lungs, brain, skin (digital ischemia), and adrenals are typical targets, and thrombocytopenia is common. Triggers include infection, surgery, and interruption of anticoagulation.
Treatment. Give "triple therapy": heparin anticoagulation, high-dose glucocorticoids, and plasma exchange and/or IVIG, while treating the trigger (e.g., antibiotics for pneumonia). Rituximab or eculizumab is used for refractory disease; mortality remains roughly one-third.