Lab result explainedInflammationLast updated November 11, 2026

Antiphospholipid antibodies: anticardiolipin and anti-beta-2 glycoprotein I results explained

Antiphospholipid antibodies (aPL) · Antiphospholipid antibody panel: anticardiolipin IgG and IgM, anti-beta-2 glycoprotein I IgG and IgM, usually with the lupus anticoagulant · reported in GPL (IgG) and MPL (IgM) units for anticardiolipin; SGU and SMU (or U/mL, CU) for anti-beta-2 glycoprotein I; some assays report an index

Antiphospholipid antibodies (aPL) are autoantibodies that bind to proteins attached to the phospholipid surfaces of cells and that raise the risk of blood clots and pregnancy complications. The two measured directly by blood test are anticardiolipin antibodies (aCL) and anti-beta-2 glycoprotein I antibodies (anti-β2GPI), each reported as IgG and IgM; the third criteria test, the lupus anticoagulant, detects the same family of antibodies by their effect on clotting and has its own page. A normal result is negative, under about 20 GPL or MPL units for anticardiolipin and under the kit cut-off (often 20 standard units) for anti-beta-2 glycoprotein I at most US labs, with cut-offs set at the 99th percentile of healthy people.

The test is ordered after an unexplained clot (especially a stroke or heart attack under 50, or a clot in an unusual site), after recurrent miscarriage or a late pregnancy loss, early severe preeclampsia, an unexplained low platelet count, a prolonged aPTT or a false positive syphilis test, and in everyone diagnosed with lupus. A positive result is graded. Low positive (20 to 39 units) is common, often transient and weakly linked to disease; moderate (40 to 79) and high (80 or more) titers, IgG rather than IgM, and antibodies that persist on a repeat test 12 weeks later carry the real risk.

Antiphospholipid antibodies are found in 1 to 5 percent of healthy people, most at low titer and transiently after infections, and in roughly 10 percent of people with a deep vein thrombosis, 11 percent of young people with a heart attack, 14 percent with stroke and 6 percent with pregnancy morbidity. Antiphospholipid syndrome (APS) is diagnosed when a persistent, clinically significant antibody is combined with a clot or a qualifying pregnancy event. The test does not diagnose APS on its own, and most low-level positives, particularly isolated IgM, never cause harm.

aPL at a glance

What it is: a panel of anticardiolipin IgG and IgM and anti-beta-2 glycoprotein I IgG and IgM, usually ordered with the lupus anticoagulant, to look for antiphospholipid syndrome.
Normal: negative, below about 20 GPL/MPL for anticardiolipin and below the kit cut-off for anti-beta-2 glycoprotein I (often 20 SGU/SMU); labs set cut-offs at the 99th percentile.
Low positive (20–39 units) is often transient and of uncertain meaning; moderate (40–79) and high (80 or more) titers, especially IgG, are what count toward antiphospholipid syndrome.
Positive on two occasions at least 12 weeks apart plus a clot or a qualifying pregnancy complication defines antiphospholipid syndrome.
Transient positives follow infections (COVID-19, hepatitis C, HIV, syphilis, EBV and others) and some drugs; a large share of low-titer first positives are gone on repeat.
Next step: repeat in 12 weeks with the lupus anticoagulant; a persistent moderate or high titer prompts a review of clot history, pregnancy plans, lupus screening and contraception.

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What aPL measures

Cardiolipin is a phospholipid, but the clinically important anticardiolipin antibodies do not bind it directly; they bind beta-2 glycoprotein I, a plasma protein that opens up and attaches to negatively charged phospholipid on activated cells. The anti-beta-2 glycoprotein I test measures antibodies to that protein directly. When these antibodies cross-link beta-2 glycoprotein I on endothelial cells, platelets and placental trophoblast, they switch on tissue factor and complement, promote platelet aggregation and impair placental function, which is how one antibody family produces both clots and pregnancy loss. Antibodies against domain I of the protein appear to be the most harmful.

Both tests are solid-phase immunoassays (ELISA or chemiluminescence) on serum. Anticardiolipin results are reported in GPL units for IgG and MPL units for IgM, a historical calibration in which 1 unit equals 1 microgram per milliliter of affinity-purified antibody; anti-beta-2 glycoprotein I results are reported in SGU and SMU or in the kit's own units. Because kits are not fully harmonized, a titer of 35 on one platform may read 25 on another, so interpretation uses the lab's own ranges and the 2023 ACR/EULAR bands of moderate (40 to 79) and high (80 or more) for IgG and IgM. IgA isotypes and antibodies to prothrombin, phosphatidylserine or annexin V are available but are not part of the classification criteria.

Timing and context affect the result. Infections commonly produce IgM anticardiolipin that fades over weeks, and a sample taken during acute illness or within days of a clot is less reliable. Unlike the lupus anticoagulant, these assays are not disturbed by anticoagulants, so they can be drawn on heparin, warfarin or a DOAC, which makes them the easier test to obtain in hospital. Pregnancy does not change the titers meaningfully.

Normal range for aPL

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (GPL (IgG) and MPL (IgM) units for anticardiolipin; SGU and SMU (or U/mL, CU) for anti-beta-2 glycoprotein I; some assays report an index)	Notes
Anticardiolipin IgG or IgM, negative	Below 20 GPL or MPL (some labs use 15)	Cut-offs are the 99th percentile of healthy donors. Values just under the line are normal.
Anticardiolipin, low positive	20–39 GPL or MPL	Common, often transient (infection, drugs) and not counted in the 2023 classification criteria. Repeat in 12 weeks.
Anticardiolipin, moderate positive	40–79 GPL or MPL	Meets the laboratory threshold for antiphospholipid syndrome if persistent. IgG at this level carries more weight than IgM.
Anticardiolipin, high positive	80 GPL or MPL and above	Strongest association with clots and pregnancy loss; high IgG scores most in the 2023 criteria.
Anti-beta-2 glycoprotein I IgG or IgM, negative	Below the kit cut-off (often below 20 SGU or SMU, or an index below 1.0)	Units differ more between kits than for anticardiolipin; use the printed range.
Anti-beta-2 glycoprotein I, positive	Above the 99th percentile; moderate 40–79 and high 80 or more on standardized units	Positive in about 40 to 80 percent of people with antiphospholipid syndrome; isolated positivity without anticardiolipin is less common and lower risk.
IgA isotypes	Reported by some labs; no agreed cut-off	Not part of the classification criteria; occasionally relevant when IgG and IgM are negative but suspicion is high.
Persistence	Positive on two occasions at least 12 weeks apart	Required before any result counts toward antiphospholipid syndrome; a large proportion of low-titer first positives are negative on repeat.
Triple positive	Lupus anticoagulant plus anticardiolipin plus anti-beta-2 glycoprotein I (usually the same isotype, medium to high titer)	Highest-risk profile: about 5 percent a year first-clot risk in carriers, and the group in which rivaroxaban proved inferior to warfarin.

Positive antiphospholipid antibodies

A positive anticardiolipin or anti-beta-2 glycoprotein I result means your immune system is making antibodies against beta-2 glycoprotein I. What that implies depends on three things: the titer, the isotype and whether it persists. A low-positive IgM anticardiolipin found once, during or after an infection, is common and usually meaningless; a moderate or high IgG titer found on two tests 12 weeks apart is a genuine prothrombotic state, whether or not you have had a clot yet.

If you have had a clot or a qualifying pregnancy event, a persistent moderate or high titer confirms antiphospholipid syndrome, which changes treatment: anticoagulation after a clot is usually indefinite rather than 3 to 6 months, warfarin is preferred to the direct oral anticoagulants (particularly in triple-positive patients, in whom rivaroxaban caused more arterial events than warfarin in the TRAPS trial), and pregnancies are managed with aspirin and heparin. If you have not had an event, you are an antibody carrier; the risk of a first clot ranges from under 1 percent a year with an isolated low-titer antibody to about 5 percent a year with triple positivity.

Antiphospholipid antibodies also explain findings that otherwise puzzle: a mildly low platelet count, a prolonged aPTT, a false positive syphilis test, livedo reticularis, thickened heart valves, migraine-like headaches and, in lupus, a higher rate of nephritis and neurological disease. In roughly a third of people with APS the antibodies occur alongside lupus or another autoimmune disease, so a positive result is usually followed by ANA, anti-dsDNA and complement testing if these have not been done.

Common causes

Primary antiphospholipid syndrome
Antibodies with clots or pregnancy complications and no other autoimmune disease; about half of all APS. Anticardiolipin IgG is positive in roughly 80 percent and anti-beta-2 glycoprotein I in 40 to 80 percent, usually together.
Systemic lupus erythematosus
30 to 40 percent of people with lupus have antiphospholipid antibodies and about 10 to 15 percent develop APS. The antibodies increase the risk of nephritis, stroke and pregnancy loss in lupus and are checked at diagnosis and before pregnancy.
Other autoimmune diseases
Rheumatoid arthritis, Sjögren's syndrome, systemic sclerosis, autoimmune hepatitis and immune thrombocytopenia (where a quarter to a third have antibodies) all carry higher rates, usually at lower titers.
Infections
Syphilis, HIV, hepatitis C, Epstein-Barr virus, parvovirus B19, Lyme disease, Q fever, leprosy, malaria and COVID-19 can induce anticardiolipin antibodies, typically IgM, low titer, not beta-2 glycoprotein I dependent, and gone within 3 months. These rarely cause clots.
Medications
Chlorpromazine, procainamide, hydralazine, quinidine, phenytoin, interferon alfa, TNF inhibitors and some antibiotics induce antibodies that usually fade after the drug is stopped and carry little thrombotic risk.
Cancer
Lymphoma, leukemia and solid tumors of the lung, kidney, ovary and breast are associated with antiphospholipid antibodies and with thrombosis; a new, high-titer antibody in an older adult prompts age-appropriate cancer screening.
Older age and healthy carriers
Low-titer antibodies are present in 1 to 5 percent of healthy adults and more often in people over 65. Most are IgM, low titer and transient; persistent moderate or high IgG titers in healthy people are uncommon (under 1 percent).
Family history and genetics
Antiphospholipid antibodies cluster in families and are more common in relatives of people with lupus or APS, pointing to HLA and other genetic susceptibility. A positive result in a relative is not, on its own, a reason to test family members who have no symptoms.
Acute illness at the time of testing
The antibodies cause clots and pregnancy complications rather than resulting from them, but testing is usually done at the time of an event, and the acute-phase response can push a borderline titer over the line. This is one more reason for the 12-week repeat.

Symptoms that can go with it

Often none; found during investigation of a clot, a pregnancy loss, a low platelet count or a prolonged aPTT
Deep vein thrombosis or pulmonary embolism, sometimes recurrent or in unusual sites (arm, abdomen, brain veins)
Stroke, transient ischemic attack or heart attack at a young age
Three or more early miscarriages, a loss after 10 weeks, severe early preeclampsia or a growth-restricted baby
Livedo reticularis, a net-like purple discoloration of the skin, or skin ulcers on the legs
Easy bruising or petechiae from a low platelet count
Heart murmur from thickened valves, migraine-like headaches, seizures or memory problems
Raynaud's phenomenon, joint pain or rashes when lupus is also present

Read it alongside

Lupus anticoagulant — the third criteria test, done by a clotting method; triple positivity is the highest-risk pattern and all three are ordered together.
aPTT — may be prolonged by a lupus anticoagulant; anticardiolipin and anti-beta-2 glycoprotein I antibodies on their own do not change it.
Platelet count — mild thrombocytopenia (usually 70,000 to 120,000 per microliter) accompanies antiphospholipid antibodies in 20 to 40 percent of APS.
PT/INR — needed if warfarin is started; some antibodies interfere with the INR, and a chromogenic factor X level is used instead.
ANA, anti-dsDNA and complement C3/C4 — screen for lupus, which coexists in about a third of people with APS and changes management.
RPR or VDRL — antiphospholipid antibodies cause a biological false positive syphilis screen with a negative confirmatory treponemal test.
D-dimer — used with imaging when a clot is suspected; not a test for antiphospholipid antibodies.
Urine albumin-to-creatinine ratio and creatinine — kidney involvement (APS nephropathy or lupus nephritis) shows up here before symptoms.

What usually happens next

A positive result is repeated at least 12 weeks later, together with the lupus anticoagulant, before any label is applied; a low-titer IgM that is negative on repeat needs no further action.
Your clinician will go through your history of clots, pregnancies (including losses and preeclampsia), migraines, livedo and lupus symptoms, and check a CBC, ANA, anti-dsDNA and complement if these have not been done.
If you have had a clot and the antibodies are persistent at moderate or high titer, expect a discussion about indefinite anticoagulation, usually warfarin with a target INR of 2 to 3, rather than a time-limited course or a DOAC.
If you have never had a clot, the conversation is about risk reduction: stopping smoking, avoiding estrogen-containing contraception, managing blood pressure and cholesterol, heparin prophylaxis around surgery and childbirth, and whether low-dose aspirin is worthwhile given your profile.
If you are planning pregnancy or pregnant, ask for early referral to a high-risk obstetric clinic; aspirin from before 16 weeks and, where indicated, low-molecular-weight heparin markedly improve outcomes.

Diet, lifestyle and natural levers with evidence

What can move aPL once the cause is known. Nothing here replaces finding the cause.

Nothing natural lowers antiphospholipid antibody titers

Strong evidence

No diet, herb or supplement has been shown in human studies to reduce anticardiolipin or anti-beta-2 glycoprotein I levels or to turn a persistent positive negative. Titers do fluctuate on their own, and transient antibodies disappear without any intervention, so a fall after starting a supplement proves nothing. Management targets the clot risk the antibodies create, not the number.

Let infections and drug effects clear before the repeat test

Strong evidence

Antibodies induced by syphilis, HIV, hepatitis C, EBV, COVID-19 and other infections, and by drugs such as chlorpromazine, hydralazine, procainamide and phenytoin, are typically IgM, low titer and gone within 3 months. Waiting the full 12 weeks, when you are well, before repeating the panel is the single most useful thing you can do to avoid being wrongly labeled; a repeat drawn during another illness is no better than the first.

Stop smoking and avoid estrogen-containing contraception and hormone therapy

Strong evidence

Smoking and estrogen each multiply the clot risk carried by antiphospholipid antibodies; in the RATIO study of young women, antibody positivity combined with oral contraceptive use or smoking raised stroke risk far beyond either alone. EULAR guidance recommends progestin-only or non-hormonal contraception, avoiding oral estrogen at menopause, stopping smoking and treating high blood pressure and cholesterol in everyone with persistent antibodies.

Arrange alternative contraception before stopping a combined pill; an unplanned pregnancy in an untreated antibody carrier carries its own risks.

Movement, hydration and weight during high-risk periods

Moderate evidence

Most first clots in antibody carriers occur around a second trigger: surgery, hospital admission, immobility, long-haul travel, pregnancy and the postpartum weeks. Walking regularly on long journeys, early mobilization after operations and asking about heparin prophylaxis in hospital and after delivery are standard recommendations. Obesity roughly doubles venous thrombosis risk in population studies, and regular physical activity lowers it, so both are worth attention in carriers.

Vitamin D if you are deficient

Preliminary evidence

Low vitamin D is more common in antiphospholipid syndrome than in controls and is associated with thrombotic rather than purely obstetric disease, and in laboratory work vitamin D blunts the tissue factor response to the antibodies. No trial has shown that replacing it prevents clots or alters titers, so a deficient level is worth correcting to normal but not a treatment for the antibodies.

Check a 25-hydroxyvitamin D level first; avoid high doses if you have kidney stones, high calcium or sarcoidosis.

When to see a clinician promptly

Emergency: a painful swollen leg, sudden breathlessness, chest pain or coughing blood (possible deep vein thrombosis or pulmonary embolism).
Emergency: sudden weakness or numbness on one side, facial droop, slurred speech, loss of vision or a sudden severe headache (possible stroke), or crushing chest pain (possible heart attack).
In pregnancy: severe headache, flashing lights, pain under the right ribs, sudden swelling or reduced fetal movements, which need same-day assessment for preeclampsia or placental problems.
Several organs failing over days (confusion, breathlessness, falling urine output, widespread rash or purple skin) in someone known to be positive: catastrophic antiphospholipid syndrome is rare but life-threatening.
Unexplained bruising, nosebleeds or bleeding gums, or a platelet count under 50,000 per microliter, which must be assessed before anticoagulation is started or continued.
Before surgery, starting hormonal treatment, long-haul travel or trying to conceive, so that prevention can be planned.

Frequently asked questions

What is a normal anticardiolipin antibody level?

Negative, which at most US labs means below 20 GPL units for IgG and below 20 MPL units for IgM (some labs use 15). Cut-offs are set at the 99th percentile of healthy people, so about 1 percent of healthy adults test just above them. A normal anti-beta-2 glycoprotein I is likewise below the kit cut-off, often 20 standard units or an index below 1.0. Both are needed, with the lupus anticoagulant, for a complete antiphospholipid screen.

Is an anticardiolipin IgM of 25 something to worry about?

Usually not on its own. A value of 20 to 39 is a low positive, the band most often produced by a recent infection, a drug or ordinary biological variation, and isolated low-titer IgM is the weakest of all antiphospholipid results for predicting clots; it is not counted at all in the 2023 classification criteria. The right response is a repeat in 12 weeks with the full panel. If the repeat is negative, the matter is closed; if it is persistent, your clinician will weigh it with your history.

What is the difference between anticardiolipin and beta-2 glycoprotein antibodies?

They detect overlapping antibodies in different ways. The anticardiolipin test coats a plate with cardiolipin and relies on beta-2 glycoprotein I in the sample bridging the antibody to it; the anti-beta-2 glycoprotein I test coats the plate with the protein itself and is therefore more specific. Most people with APS are positive on both. Isolated anti-beta-2 glycoprotein I positivity is less common and lower risk; isolated anticardiolipin positivity, especially IgM, is often infection related.

Can antiphospholipid antibodies go away?

Transient antibodies from infections and drugs usually clear within 3 months, which is why nothing is concluded from a single result. Persistent antibodies tend to remain for years, although titers rise and fall, and a minority of people become negative over long follow-up. Treatment decisions are based on persistence and clinical events rather than on a single later negative, since antibodies can reappear.

I tested positive but have never had a clot. Do I need treatment?

Not usually anticoagulation. Your risk depends on the profile: an isolated low-titer antibody carries a small risk (under 1 percent a year), while triple positivity carries about 5 percent a year. EULAR suggests low-dose aspirin for carriers with a high-risk profile or lupus, and for everyone, stopping smoking, avoiding estrogen-containing contraception, managing blood pressure and cholesterol, and using heparin prophylaxis around surgery and childbirth. Low-dose aspirin was associated with about half the rate of a first clot in a meta-analysis of observational studies, though trial evidence is thin.

Do positive antiphospholipid antibodies mean I have lupus?

No. About a third of people with antiphospholipid syndrome have lupus or another autoimmune disease; the rest have primary APS, a transient antibody or an isolated finding. The reverse is more common: 30 to 40 percent of people with lupus carry antiphospholipid antibodies. Because the overlap matters for treatment and pregnancy, a positive result usually prompts an ANA, anti-dsDNA and complement check if these have not been done.

How do antiphospholipid antibodies affect pregnancy and IVF?

Persistent antibodies with a history of losses define obstetric APS, and treatment with low-dose aspirin plus prophylactic heparin raises live birth rates to around 70 to 80 percent, from roughly 40 to 50 percent untreated in older series. Women without previous losses are usually given aspirin and closer monitoring for preeclampsia and growth restriction. For IVF, the American Society for Reproductive Medicine does not recommend routine antiphospholipid testing for implantation failure, because treatment has not been shown to improve IVF success in women without a history of clots or losses.

Can I have the test while taking blood thinners?

Yes for anticardiolipin and anti-beta-2 glycoprotein I, which are immunoassays unaffected by heparin, warfarin or the direct oral anticoagulants. The lupus anticoagulant, by contrast, is a clotting test and is unreliable on anticoagulants, particularly apixaban, rivaroxaban and dabigatran. In hospital the antibody panel is therefore often drawn first and the lupus anticoagulant deferred or timed to a drug trough.

References

Antiphospholipid antibodies. MedlinePlus, US National Library of Medicine (2023)
Miyakis S, Lockshin MD, Atsumi T, et al.. International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). Journal of Thrombosis and Haemostasis (2006)
Barbhaiya M, Zuily S, Naden R, et al.. The 2023 ACR/EULAR antiphospholipid syndrome classification criteria. Arthritis & Rheumatology (2023)
Garcia D, Erkan D. Diagnosis and management of the antiphospholipid syndrome. New England Journal of Medicine (2018)
Tektonidou MG, Andreoli L, Limper M, et al.. EULAR recommendations for the management of antiphospholipid syndrome in adults. Annals of the Rheumatic Diseases (2019)
Andreoli L, Chighizola CB, Banzato A, Pons-Estel GJ, Ramire de Jesus G, Erkan D. Estimated frequency of antiphospholipid antibodies in patients with pregnancy morbidity, stroke, myocardial infarction, and deep vein thrombosis: a critical review of the literature. Arthritis Care & Research (2013)
Pengo V, Denas G, Zoppellaro G, et al.. Rivaroxaban vs warfarin in high-risk patients with antiphospholipid syndrome. Blood (2018)
Pengo V, Ruffatti A, Legnani C, et al.. Incidence of a first thromboembolic event in asymptomatic carriers of high-risk antiphospholipid antibody profile: a multicenter prospective study. Blood (2011)
de Groot PG, Urbanus RT. The significance of autoantibodies against β2-glycoprotein I. Blood (2012)
Hamulyák EN, Scheres LJ, Marijnen MC, Goddijn M, Middeldorp S. Aspirin or heparin or both for improving pregnancy outcomes in women with persistent antiphospholipid antibodies and recurrent pregnancy loss. Cochrane Database of Systematic Reviews (2020)
American College of Obstetricians and Gynecologists. Practice Bulletin No. 132: Antiphospholipid syndrome. Obstetrics & Gynecology (2012)
Antiphospholipid antibody syndrome (APS). Merck Manual Professional Version (2024)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.