Antiphospholipid syndrome

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Antiphospholipid syndrome is an acquired autoimmune thrombophilia characterised by objectively confirmed arterial, venous or small-vessel thrombosis, or defined pregnancy morbidity, together with persistent lupus anticoagulant, anticardiolipin or anti-beta-2-glycoprotein-I antibodies. It can be primary or accompany SLE. The diagnosis should explain an event, not merely a blood result. Antibody profile, event type, reversible provoking factors and cardiovascular risk determine recurrence risk and treatment.

Laboratory interpretation is unusually vulnerable to error. Acute thrombosis, inflammation and anticoagulants can distort lupus-anticoagulant testing, while low-titre anticardiolipin may be transient after infection. Send the complete panel, document anticoagulant exposure, involve the coagulation laboratory when results conflict and repeat at least twelve weeks later. A persistently high-risk triple-positive profile carries different implications from a single low result. Do not interrupt life-preserving anticoagulation without a supervised testing plan.

After a first venous thrombotic APS event, warfarin at INR 2.0–3.0 is standard secondary prevention for most patients, with duration shaped by whether the event was provoked and the antibody risk. Arterial and recurrent events need specialist review of diagnosis, adherence, time in therapeutic range and competing atherosclerotic or embolic sources before raising intensity or adding antiplatelet therapy. DOACs are particularly unsuitable in triple-positive APS. Good anticoagulation control is a clinical intervention, not an administrative detail.

Pregnancy management differs from non-pregnant anticoagulation. Warfarin is generally replaced before or early in pregnancy; obstetric APS without prior thrombosis commonly receives low-dose aspirin plus prophylactic LMWH, while a history of thrombosis normally requires therapeutic LMWH. Postpartum risk remains high. Catastrophic APS is rare but must be recognised rapidly and distinguished from TTP, sepsis, DIC, complement-mediated HUS and severe pre-eclampsia.

Key points

APS requires a relevant clinical event and persistent antiphospholipid antibodies; an isolated positive test without thrombosis or defined pregnancy morbidity is not the syndrome.
Order lupus anticoagulant, IgG and IgM anticardiolipin and IgG and IgM anti-beta-2-glycoprotein-I, then confirm clinically relevant positivity at least twelve weeks later.
Anticoagulants interfere with lupus-anticoagulant assays, so record the exact agent, timing and laboratory strategy; never stop necessary anticoagulation unsafely just to obtain a cleaner test.
Treat an acute VTE through the standard emergency pathway, then use a vitamin-K antagonist with target INR 2.0–3.0 for most first venous APS thromboses; duration is commonly long term after an unprovoked event.
Avoid rivaroxaban in triple-positive APS and generally avoid DOAC substitution in high-risk APS because recurrent events, particularly arterial events, are increased.
For arterial APS, recurrent thrombosis or bleeding constraints, haematology individualises warfarin intensity, possible low-dose aspirin and investigation of adherence, INR control and competing mechanisms.
Obstetric APS without previous thrombosis is usually managed with low-dose aspirin plus prophylactic LMWH from a positive pregnancy test, continuing postpartum prophylaxis according to risk; previous thrombotic APS requires therapeutic anticoagulation.
Catastrophic APS is a critical-care diagnosis: therapeutic heparin, high-dose glucocorticoid and plasma exchange or IVIG are combined while infection or another trigger is treated.
Manage smoking, pressure, lipids, diabetes, obesity, immobility and oestrogen exposure; antibody status does not remove ordinary vascular prevention.
Do not use the 2023 ACR/EULAR classification score as a diagnostic gate; classification serves research and clinical judgement remains essential.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Primary or autoimmune-associated APS

APS may occur alone or with systemic lupus erythematosus and other autoimmune disease; persistent autoantibodies interact with vascular and complement pathways rather than acting as a sufficient cause alone.

Acquired thrombotic triggers

Surgery, immobility, pregnancy, oestrogen exposure, smoking, infection and cardiovascular risk provide a second hit that converts antibody-associated susceptibility into clinical thrombosis.

High-risk antibody profile

Lupus anticoagulant, triple positivity, persistent high titres and previous thrombosis indicate greater risk than an isolated transient low-titre anticardiolipin result.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Endothelial and cellular activation
Antibodies targeting beta-2-glycoprotein-I-associated complexes activate endothelium, monocytes and platelets, increase tissue factor and reduce anticoagulant surface properties.
2
Complement and coagulation amplification
Complement activation and procoagulant signalling reinforce thrombin generation and vascular inflammation, particularly at the placenta and during systemic triggers.
3
Placental dysfunction
Thrombosis, complement injury and impaired trophoblast function cause implantation failure, fetal loss, growth restriction and early severe placental insufficiency.
4
Catastrophic microvascular occlusion
A powerful trigger can produce rapidly disseminated small-vessel thrombosis and multiorgan failure, often with thrombocytopenia and haemolysis despite variable large-vessel involvement.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Venous thrombotic phenotype

Unprovoked or recurrent DVT, pulmonary embolism or unusual-site splanchnic, cerebral or retinal venous thrombosis can indicate APS after objective confirmation.

Arterial thrombotic phenotype

Stroke, TIA, myocardial, limb or retinal arterial occlusion in a younger patient or alongside SLE warrants targeted aPL testing and ordinary vascular work-up.

Obstetric phenotype

Recurrent early losses, fetal death, or premature birth from severe placental insufficiency or pre-eclampsia may meet the clinical pregnancy pattern after alternative causes are assessed.

Non-criteria clues

Livedo racemosa, thrombocytopenia, valve disease, chorea, skin ulcers and APS nephropathy increase suspicion but do not independently establish the syndrome.

Catastrophic patternRed flag

Three or more organ systems developing thrombosis over about one week with small-vessel occlusion suggests catastrophic APS and demands immediate combination care.

Major anticoagulant bleedingRed flag

Neurological change, haemodynamic compromise, melaena, haematemesis or expanding bleeding on anticoagulation requires urgent reversal and source-control assessment.

Red flags requiring action

New unilateral weakness, chest pain, hypoxia, painful swollen limb, mesenteric pain or limb ischaemia requires the appropriate acute arterial or venous thrombosis pathway.
Thrombosis in several organs within one week with laboratory or histological small-vessel occlusion requires urgent catastrophic APS assessment.
Pregnancy with pain, bleeding, hypertension, fetal compromise or thrombosis requires same-day obstetric and haematology review rather than routine anticoagulation adjustment.
Thrombocytopenia with schistocytes, haemolysis, fever, neurological or renal injury may represent TTP or another TMA and needs emergency plasma-exchange pathway assessment.
Major bleeding, head injury, melaena or haemodynamic change during anticoagulation requires immediate severity assessment and agent-specific reversal planning.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Objective thrombosis imaging or histologyFirst step
Why
Confirm the clinical event and define its vascular territory, severity and alternative mechanism.
Interpretation and limitations
Use the standard acute pathway for ultrasound, CTPA, CT, MRI, angiography or tissue; a historical self-reported clot is insufficient without corroboration.
02
Lupus anticoagulant
Why
Identify the phospholipid-dependent coagulation-inhibitor pattern most strongly associated with thrombosis.
Interpretation and limitations
Anticoagulants and acute-phase effects can cause false results; the laboratory uses screening, mixing and phospholipid-confirmation steps and needs complete treatment information.
03
Anticardiolipin and anti-beta-2-glycoprotein-I
Why
Complete the laboratory risk profile using IgG and IgM solid-phase assays.
Interpretation and limitations
Medium or high persistent titres carry more weight than transient low levels; repeat clinically relevant positivity after at least twelve weeks.
04
FBC, coagulation screen, film and haemolysis tests
Why
Detect thrombocytopenia, DIC, TMA, bleeding risk and a competing haematological emergency.
Interpretation and limitations
A prolonged APTT can coexist with thrombosis and does not imply protection; schistocytes and severe thrombocytopenia require urgent TTP or TMA assessment.
05
Cause-directed thrombosis assessment
Why
Find provoking factors and alternative causes that modify treatment duration and recurrence prevention.
Interpretation and limitations
Assess cancer, atrial fibrillation, atherosclerosis, nephrotic syndrome, myeloproliferative disease, oestrogen, pregnancy, surgery and immobility as clinically indicated.
06
Pregnancy and placental assessment
Why
Define current maternal thrombosis, hypertension, fetal growth and placental risk.
Interpretation and limitations
Coordinate ultrasound, pressure, urine, blood count and liver or renal tests with obstetric medicine; distinguish APS from pre-eclampsia and other loss causes.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Inherited or acquired thrombophilia

Factor V Leiden, prothrombin variants, antithrombin, protein C or S deficiency, malignancy, nephrotic syndrome and myeloproliferative disease may explain thrombosis.

TTP and other microangiopathies

Severe ADAMTS13 deficiency, shiga-toxin HUS, complement-mediated HUS, malignant hypertension and pregnancy-related TMA cause thrombocytopenia and organ injury through distinct emergency mechanisms.

Physiological pregnancy loss

Most isolated early miscarriages are chromosomal or anatomical rather than APS; obstetric history must meet a defined clinical pattern before antibody results are causal.

Transient antibody positivity

Infection, medicines and assay interference can produce temporary or misleading aPL results; persistence and a credible clinical event prevent overdiagnosis.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Confirmatory diagnostic sequenceLink event, profile and persistenceFirst stepConfirmatoryThrombosis or a qualifying pregnancy history raises suspicion of APS.

1Confirm the event objectively, record provoking factors and obtain a complete obstetric, autoimmune, medicine, bleeding and family history.
2Send lupus anticoagulant, anticardiolipin and anti-beta-2-glycoprotein-I with exact anticoagulant details and coagulation-laboratory advice.
3Repeat relevant positivity after at least twelve weeks, then integrate antibody risk and clinical phenotype rather than diagnosing from one result.

02First venous thrombosisTreat acutely then prevent recurrenceAn objectively confirmed DVT or pulmonary embolism occurs with a high suspicion or diagnosis of APS.

1Deliver standard acute anticoagulation and stabilisation, sampling aPL before anticoagulation when this is safe but never delaying treatment for a dangerous clot.
2Transition most confirmed thrombotic APS patients to warfarin with target INR 2.0–3.0 and specialist education, interactions and monitoring.
3Decide duration from provocation, antibody profile and recurrence or bleeding risk; an unprovoked high-risk APS event usually leads to indefinite treatment.

03Recurrent or arterial eventAudit treatment before intensifyingThrombosis occurs despite prescribed therapy or the index event is arterial.

1Confirm a new event and review missed doses, INR history, interactions, malignancy, atherosclerosis, embolic sources and whether APS diagnosis remains secure.
2Optimise time in range and manage ordinary vascular risks before a specialist considers INR 3.0–4.0 or INR 2.0–3.0 plus low-dose aspirin for selected patients.
3Avoid routine DOAC substitution in triple-positive or arterial APS and create an individual bleeding and recurrence plan.

04Catastrophic escalationCombine anticoagulation and immune treatmentEscalationRapid multiorgan small-vessel thrombosis suggests catastrophic APS.

1Admit to critical care, obtain organ imaging and tissue where feasible, and investigate sepsis, DIC, TTP, complement-mediated HUS and pregnancy-related TMA concurrently.
2Start therapeutic unfractionated or low-molecular-weight heparin when not contraindicated and treat infection, surgery, malignancy or medicine interruption as the precipitant.
3Add specialist high-dose glucocorticoid and plasma exchange or IVIG; consider additional immunomodulation for refractory disease or active SLE.

Key medicines and prescribing safety5 treatments · regimens, roles and cautions

Preferred oral secondary prevention for most thrombotic APS, particularly high-risk antibody profiles.

Warfarin for venous thrombotic APS

Start through an anticoagulation service with heparin overlap when treating acute VTE, individualising the daily dose to achieve INR 2.0–3.0; continue long term when recurrence risk outweighs bleeding risk.

Review interactions, alcohol, adherence, diet consistency, bleeding and time in therapeutic range. It is teratogenic and generally avoided during pregnancy; major bleeding requires urgent agent-specific reversal. Do not use a fixed milligram dose without INR monitoring.

Used with prophylactic heparin for obstetric APS and selectively for high-risk asymptomatic or arterial phenotypes after specialist benefit-risk assessment.

Low-dose aspirin

Use 75–100 mg orally once daily when the APS or pregnancy pathway specifically indicates antiplatelet treatment; UK obstetric protocols may use up to 150 mg at night for pre-eclampsia prevention.

Assess ulcer, bleeding, asthma, renal disease and concurrent anticoagulants. Aspirin alone is inadequate secondary prevention after most APS venous thromboses and should not be added to warfarin routinely.

Provides pregnancy-compatible anticoagulation for obstetric APS and for patients with previous thrombosis when warfarin is unsuitable.

Enoxaparin in pregnancy

A common prophylactic regimen is 40 mg subcutaneously once daily; previous thrombotic APS usually needs weight-based therapeutic dosing such as 1 mg/kg subcutaneously every twelve hours under obstetric-haematology monitoring.

Adjust for weight, renal impairment and bleeding risk, check platelets when HIT risk exists, and plan timing around labour, neuraxial anaesthesia and postpartum restart. Prolonged exposure affects bone and bruising.

Rapidly titratable anticoagulation within emergency combination treatment for catastrophic APS or severe thrombosis requiring procedures.

Unfractionated heparin in catastrophic APS

Give an intravenous weight-based therapeutic infusion according to the hospital UFH protocol, using anti-Xa rather than APTT monitoring when lupus anticoagulant makes APTT unreliable.

Assess active bleeding, recent surgery, HIT history and platelet trend; renal clearance is less limiting than with LMWH, but monitoring and reversal must be available.

Adjunct immunomodulation with anticoagulation and glucocorticoid in catastrophic APS when selected by the multidisciplinary team.

IVIG for catastrophic APS

A common specialist regimen is total intravenous immunoglobulin 2 g/kg divided over two to five days, generally after or coordinated with plasma exchange.

Dose by the agreed body-weight method and assess thrombosis, hyperviscosity, renal impairment, fluid load, haemolysis, headache and aseptic meningitis; product choice and infusion rate matter.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Recurrent venous or arterial thrombosis

Recurrent DVT, pulmonary embolism, stroke, myocardial, retinal, splanchnic or limb thrombosis can occur, especially after interruption, subtherapeutic anticoagulation or high-risk exposures.

Pregnancy morbidity

Recurrent fetal loss, placental insufficiency, fetal growth restriction, pre-eclampsia and preterm birth may recur without a planned obstetric antithrombotic pathway.

Catastrophic APS

Disseminated small-vessel thrombosis causes renal, respiratory, cerebral, cardiac, adrenal, skin and gastrointestinal failure with high mortality despite combination treatment.

Anticoagulant harm

Major bleeding, intracranial haemorrhage, heparin-induced thrombocytopenia, osteoporosis with prolonged heparin and fetal warfarin toxicity require continuing risk review.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

For warfarin, track INR, time in therapeutic range, missed doses, interactions, bleeding and recurrent symptoms; increase testing after medicines or health status changes.
Reassess cardiovascular risks, smoking, weight, mobility, oestrogen exposure, surgery and travel because modifiable second hits remain important.
Do not repeat aPL titres routinely to decide anticoagulant dose; repeat only when persistence or a specific clinical decision remains unresolved.
During pregnancy monitor platelets, renal function, blood pressure, proteinuria, fetal growth and placental function, and document delivery and postpartum anticoagulation timing.
After any recurrence audit the objective event, adherence and anticoagulation quality before escalating intensity or combining antithrombotics.
Give written advice for bleeding, head injury, pregnancy, procedures, missed doses and symptoms of recurrent venous or arterial thrombosis.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

A prolonged APTT can mislead

Lupus anticoagulant prolongs phospholipid-dependent tests in vitro while associating with thrombosis in vivo, so it is not an endogenous anticoagulant.

Classification is not diagnosis

Research classification improves cohort specificity but can exclude clinically credible APS and should not override expert assessment of an individual event.

Triple positivity changes treatment confidence

Persistent positivity across all three assay groups identifies a high-risk phenotype in which DOAC harm signals are strongest.

Obstetric injury is not only clot

Complement activation and trophoblast dysfunction contribute to placental disease, explaining benefit from combined aspirin and heparin beyond simple large-vessel thrombosis.

Recurrence demands mechanism review

A new stroke on warfarin may reflect poor INR control, atherosclerosis, valve disease, embolism or small-vessel APS and cannot be solved by reflex dose escalation.

11Common pitfallsFrequent interpretation and management errors.

01
Diagnosing APS from one low-titre antibody result in a patient without an objective clinical event.
02
Repeating lupus-anticoagulant testing without recording DOAC, heparin or warfarin exposure and then treating assay interference as biology.
03
Switching a triple-positive patient from warfarin to a DOAC for convenience without specialist review.
04
Raising INR target after an alleged recurrence before confirming the clot, adherence and time in therapeutic range.
05
Using aspirin alone after a confirmed APS venous thrombosis when therapeutic anticoagulation is indicated.
06
Stopping anticoagulation for pregnancy, biopsy or surgery without a documented bridging, neuraxial and restart plan.