Does a positive anticardiolipin test mean I have antiphospholipid syndrome?

No. A positive result must be moderate-to-high titre, confirmed on a second occasion at least twelve weeks later, and accompanied by a qualifying clinical event — thrombosis or pregnancy morbidity — to meet APS criteria. Low-titre or transient positivity is often clinically insignificant.

Why is repeat testing required?

Infections and other triggers can cause transient anticardiolipin antibodies that resolve within weeks. The revised Sapporo criteria require persistent positivity at twelve weeks to distinguish pathogenic from transient antibodies and reduce false-positive diagnoses.

Can anticardiolipin antibodies be positive without thrombosis?

Yes. Some patients with systemic lupus erythematosus or other autoimmune diseases have persistent antibodies without thrombotic events. The antibodies confer risk but do not mandate anticoagulation in the absence of a clinical event or very high-risk profile.

Is fasting required for an anticardiolipin antibody test?

No. The test does not require fasting, though a fasting sample may be requested if it is part of a panel that includes lipid-sensitive assays.

Should anticardiolipin antibodies be tested with other antiphospholipid antibodies?

Yes. A complete antiphospholipid panel includes lupus anticoagulant and anti-beta-2-glycoprotein I antibodies, each of which adds independent diagnostic and prognostic information. Lupus anticoagulant in particular carries the highest thrombotic risk.

Do results differ between laboratories?

Yes. Anticardiolipin assays vary in calibration and reagent composition, so serial results should be compared within the same laboratory. Titre thresholds for positivity are standardised but not perfectly harmonised across platforms.

How long do anticardiolipin antibody results take?

Results are typically available within three to five business days from specimen receipt at LabCorp.

What is the role of anticardiolipin testing in pregnancy loss?

Persistent moderate-to-high-titre anticardiolipin antibodies are one laboratory criterion for obstetric antiphospholipid syndrome in women with recurrent early pregnancy loss or unexplained late fetal death. The test guides decisions about aspirin, heparin, or other interventions in subsequent pregnancies.

BML-CAR-040cardiovascularUSA

Anticardiolipin Antibodies

Q: What is the difference between IgG, IgM, and IgA anticardiolipin antibodies?

IgG is the most clinically relevant isotype, associated with thrombosis and obstetric morbidity in antiphospholipid syndrome. IgM can also contribute to APS but is more often transient. IgA is not part of consensus criteria but may be tested when IgG and IgM are negative yet clinical suspicion remains high.

The anticardiolipin antibodies test measures serum IgG, IgM, and IgA autoantibodies against cardiolipin–beta-2-glycoprotein I complexes. Persistent moderate-to-high-titre IgG or IgM positivity, confirmed at least twelve weeks apart, is a laboratory criterion for antiphospholipid syndrome when combined with thrombosis or pregnancy morbidity, and is interpreted alongside lupus anticoagulant and…

Biomarkers

Blood (serum)

Sample

3–5 business days

Turnaround

Fasting

What is the aCL test?

The Anticardiolipin Antibodies test measures serum IgG, IgM, and sometimes IgA autoantibodies directed against cardiolipin–beta-2-glycoprotein I complexes. It is a cornerstone laboratory investigation for antiphospholipid syndrome, ordered to evaluate unexplained thrombosis, recurrent pregnancy loss, or thrombocytopaenia when an autoimmune prothrombotic state is suspected. Anticardiolipin antibodies do not exist in isolation within the antiphospholipid antibody family: they are interpreted alongside lupus anticoagulant and anti-beta-2-glycoprotein I antibodies, each of which adds independent information. A single positive result carries little diagnostic weight — the revised Sapporo criteria require moderate-to-high-titre positivity confirmed on a second occasion at least twelve weeks later, combined with a qualifying clinical event. Practitioners order the test to screen for APS, to investigate prolonged aPTT that may reflect lupus anticoagulant, and to risk-stratify patients with systemic lupus erythematosus or other autoimmune conditions.

What does the aCL test measure?

aCL IgG(Anticardiolipin IgG)

Range: <20 GPL U/mL negative; 20–80 low-to-moderate positive; >80 high positive.Optimal: Negative (<20 GPL U/mL). Persistent moderate-to-high positivity (≥40 GPL U/mL on two occasions ≥12 weeks apart) supports a diagnosis of APS in the correct clinical context.GPL U/mL / CU

IgG anticardiolipin antibodies are autoantibodies directed against phospholipid-binding proteins, principally beta-2-glycoprotein I, rather than cardiolipin alone. They arise through molecular mimicry or loss of tolerance and persist for weeks to months, making IgG the most specific isotype for chronic thrombotic risk. The epitopes recognised differ between transient infection-associated and persistent pathogenic antibodies.

IgG aCL is the most clinically relevant isotype for antiphospholipid syndrome, associated with both arterial and venous thrombosis and obstetric complications. Moderate-to-high titres that persist on repeat testing at least twelve weeks apart are one of the laboratory criteria for APS; low-titre or transient positivity carries less thrombotic risk and may reflect infection or other triggers.

aCL IgM(Anticardiolipin IgM)

Range: <20 MPL U/mL negative; 20–80 low-to-moderate positive; >80 high positive.Optimal: Negative (<20 MPL U/mL). Persistent moderate-to-high positivity on repeat testing ≥12 weeks apart may contribute to APS classification.MPL U/mL / CU

IgM anticardiolipin antibodies represent an earlier or transient immune response to phospholipid–protein complexes. They are more commonly transient and infection-associated than IgG, though persistent moderate-to-high IgM can contribute to APS classification in some patients.

IgM aCL is less specific for antiphospholipid syndrome than IgG. Isolated IgM positivity, especially at low titre, is often transient and may follow infection; persistent moderate-to-high IgM can support an APS diagnosis when clinical criteria are met, but the prognostic weight is lower than for IgG.

aCL IgA(Anticardiolipin IgA)

Range: <20 APL U/mL negative; 20–80 low-to-moderate positive; >80 high positive.Optimal: Negative (<20 APL U/mL). Clinical significance of isolated IgA positivity is uncertain and should be correlated with other antiphospholipid antibodies and clinical features.APL U/mL / CU

IgA anticardiolipin antibodies are less well characterised than IgG or IgM. They are detected in a subset of patients with clinical features of APS who are negative for IgG and IgM, though their independent pathogenicity remains debated.

IgA aCL is not part of the revised Sapporo classification criteria for APS but may be tested when clinical suspicion is high and IgG/IgM are negative. Some evidence suggests IgA can be associated with thrombotic events, but routine measurement is not universally recommended.

Why is the aCL test ordered?

Investigation of unprovoked venous thromboembolism, particularly in patients under fifty years or with recurrent events
Evaluation of arterial thrombosis — stroke, myocardial infarction, or peripheral arterial occlusion — in the absence of traditional vascular risk factors
Work-up of recurrent early pregnancy loss (three or more consecutive losses before ten weeks) or unexplained late fetal death
Assessment of thrombocytopaenia or livedo reticularis in a patient with suspected antiphospholipid syndrome
Screening patients with systemic lupus erythematosus for antiphospholipid antibodies, which are present in roughly one-third and confer thrombotic risk
Evaluation of a prolonged activated partial thromboplastin time (aPTT) that does not correct on mixing studies, suggesting lupus anticoagulant
Risk stratification in selected patients with autoimmune disease, unexplained neurological events, or catastrophic multi-organ thrombosis

Sample collection and turnaround

Sample type

Blood (serum)

Fasting required

Collection method

Venipuncture at patient service centre

Turnaround

3–5 business days

Collection notes

Phlebotomy at LabCorp draw site; no special preparation. For follow-up confirmation testing, collect at least twelve weeks after the initial positive result. Avoid grossly haemolysed specimens.

Specimen requirements

Minimum 1 mL serum (gel separator acceptable). Separate from cells promptly; serum may be refrigerated or frozen if testing is delayed. Note the performing laboratory for serial comparability.

What can affect aCL results?

elevatesTransient positivity following infection

Viral and bacterial infections can induce low-to-moderate-titre anticardiolipin antibodies that resolve within weeks to months; repeat testing at twelve weeks distinguishes transient from persistent positivity.

elevatesCertain medications (procainamide, hydralazine, phenytoin)

Drug-induced lupus can produce antiphospholipid antibodies, though these are usually low-titre and less commonly associated with thrombosis; a medication history is essential.

variableAssay and reagent variation

Anticardiolipin assays differ in the source of cardiolipin, the addition of beta-2-glycoprotein I, and calibration, so results are not directly comparable across platforms; serial samples should be run in the same laboratory.

variableAnticoagulation therapy

Warfarin and heparin do not interfere with anticardiolipin ELISA, but direct oral anticoagulants may affect some assays; timing relative to dosing should be consistent for serial monitoring.

variableHaemolysis and lipaemia

Grossly haemolysed or lipaemic specimens can interfere with colorimetric immunoassays and should be rejected; a fasting sample reduces lipaemia when the panel includes lipid-sensitive tests.

What do aCL results mean?

Elevated

Medium-to-high titre IgG or IgM anticardiolipin antibodies support a diagnosis of antiphospholipid syndrome when persistent and accompanied by thrombosis or pregnancy morbidity. IgG isotype and higher titres carry greater thrombotic significance than IgM. A single positive result is not diagnostic — classification requires persistence on repeat testing at least 12 weeks apart, interpreted alongside lupus anticoagulant and anti-beta-2-glycoprotein I.

Normal

A negative result in a patient without thrombosis or pregnancy morbidity indicates low probability of antiphospholipid syndrome. In the context of a suggestive clinical history, negativity does not rule out the syndrome and the remaining antiphospholipid criteria antibodies should be measured.

Reduced

Low or negative anticardiolipin antibodies reduce the likelihood of antiphospholipid syndrome but do not exclude it, as the syndrome can be driven by lupus anticoagulant or anti-beta-2-glycoprotein I antibodies with negative anticardiolipin. Isolated seronegative presentations warrant the full antiphospholipid panel.

Note: Transient low-titre elevations occur with infection, medications, and advancing age without conferring thrombotic risk, which is why persistence at 12 weeks is required for classification. Results are assay- and isotype-specific and are not interchangeable between laboratories; interpret as part of the full antiphospholipid antibody panel rather than in isolation.

Frequently asked questions

IgG is the most clinically relevant isotype, associated with thrombosis and obstetric morbidity in antiphospholipid syndrome. IgM can also contribute to APS but is more often transient. IgA is not part of consensus criteria but may be tested when IgG and IgM are negative yet clinical suspicion remains high.

How to access the aCL test

The aCL reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.

Create Free Account

About this page

Published: October 7, 2026

Last reviewed: October 7, 2026

References:

Miyakis S, et al. International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). J Thromb Haemost. (2006)
Pengo V, et al. Update of the guidelines for lupus anticoagulant detection. J Thromb Haemost. (2009)
Garcia D, Erkan D. Diagnosis and management of the antiphospholipid syndrome. N Engl J Med. (2018)
Devreese KMJ, et al. Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis. J Thromb Haemost. (2020)
Andreoli L, et al. EULAR recommendations for women's health and the management of family planning, assisted reproduction, pregnancy and menopause in patients with systemic lupus erythematosus and/or antiphospholipid syndrome. Ann Rheum Dis. (2017)

This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.