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Anticardiolipin Antibody IgM

Measures IgM antibodies to cardiolipin, a fat in cell membranes. Levels above your lab's cutoff are one piece of testing for antiphospholipid antibodies.

AI-verified against cited sources. Awaiting expert reviewAlso called aCL IgM, IgM aCLLOINC 3182-3

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What it is

Cardiolipin is a fat found in cell membranes. Anticardiolipin (aCL) antibodies are autoantibodies, which means your immune system makes antibodies against it. They belong to a group called antiphospholipid antibodies, along with the lupus anticoagulant and anti-beta-2 glycoprotein I antibodies. This test measures the IgM type, usually in MPL units. Many labs also report a call such as negative, indeterminate or positive.

Why it matters

Antiphospholipid antibodies are linked to antiphospholipid syndrome (APS), an autoimmune condition in which blood clots form more easily and some pregnancy complications are more likely. Some people with these antibodies never have a clot. The antibodies can also be temporary, so one positive result is not enough. Lab criteria look for moderate or high levels on two tests at least 12 weeks apart. In the 2023 classification criteria, a positive IgM result alone counts for less than a positive IgG result (1 point compared with at least 4). Your clinician reads this result with the IgG aCL, lupus anticoagulant and anti-beta-2 glycoprotein I tests.

If it’s high

A level above your lab's cutoff means your blood has IgM antibodies to cardiolipin. Low levels can be seen in healthy people and are sometimes temporary. Higher levels that persist on repeat testing carry more weight. A positive result by itself does not mean you have antiphospholipid syndrome. Your clinician looks at your history and your other antiphospholipid tests.

Common causes

  • Older age, or no known condition at all, since low levels can be found in healthy people
  • Infections such as HIV, hepatitis C, hepatitis B and Epstein-Barr virus, often temporary
  • Lupus and other autoimmune conditions (20% to 30% of people with lupus have antiphospholipid antibodies)
  • Antiphospholipid syndrome, an autoimmune condition linked to blood clots and some pregnancy complications
  • Some cancers and nervous system conditions

What can help

  • Ask your clinician whether to repeat the test in about 12 weeks to see if the result persists
  • Ask whether IgG anticardiolipin, lupus anticoagulant and anti-beta-2 glycoprotein I tests should be checked too
  • Tell your clinician about any past blood clots, pregnancy losses or lupus, since these change how the result is read
  • If you use or plan to use estrogen-containing birth control, talk with your clinician about this result first

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–12 MPL U/mL. Source: ARUP Laboratories, Clinical and Applied Thrombosis/Hemostasis.

ARUP Laboratories interpretation (semi-quantitative ELISA)

  • Negativebelow 13 MPL U/mL
  • Indeterminate13 to under 20 MPL U/mL
  • Low to moderately positive20 to under 81 MPL U/mL
  • High positive81 and above MPL U/mL

2023 ACR/EULAR APS classification thresholds (standardized ELISA; research classification, not a diagnosis)

  • Below the moderate-positive levelbelow 40 MPL U/mL
  • Moderate positive (40 to 79 units)40 to under 80 MPL U/mL
  • High positive (80 units or more)80 and above MPL U/mL

What can affect your result

  • Infections such as HIV, hepatitis C, hepatitis B or Epstein-Barr virus can cause temporary positive results
  • Age: these antibodies are more often found in older adults, with or without a known condition
  • Pregnancy: in one review, about 6 in 100 pregnant women without antiphospholipid syndrome had antiphospholipid antibodies
  • Other autoimmune conditions, especially lupus
  • Test kit and lab cutoffs differ, so compare results only from the same lab and method
  • Hemolyzed, heat-inactivated, lipemic or contaminated samples can be rejected by the lab, and ARUP accepts serum only (ARUP specimen requirements).
  • Infections such as HIV, hepatitis C, hepatitis B and Epstein-Barr virus can cause temporary positive results, which is why a first positive is usually repeated.
  • ELISA results in GPL or MPL units and chemiluminescent results (U/mL or CU) use different scales and cutoffs, so they can't be compared directly.

Slow and variable. Levels can come and go, so what matters most is whether a result persists on a repeat test at least 12 weeks later.

When to talk to a clinician

A positive aCL result is not an emergency by itself. Get urgent care for signs of a blood clot, such as sudden pain, swelling, warmth or redness in an arm or leg, chest pain or shortness of breath, or sudden weakness, trouble speaking, vision changes or a severe headache. Call 911 or your local emergency number for these. If you are pregnant or planning pregnancy, tell your care team about this result early.

Questions you could ask

  • Why was this test ordered for me?
  • Should we repeat this test in about 12 weeks to see if it persists?
  • Should IgG anticardiolipin, lupus anticoagulant and anti-beta-2 glycoprotein I also be checked?
  • Does this result matter for pregnancy planning, surgery or long travel for me?
  • Could a recent infection explain this result?

Questions people ask

Does a positive IgM anticardiolipin result mean I have antiphospholipid syndrome?

No, not by itself. Clinicians look for antibodies that persist on repeat tests at least 12 weeks apart, along with a history of blood clots or certain pregnancy problems. Some people with a positive test never have these problems, and antibodies can appear for a while after an infection.

Why does my clinician want to repeat the test in 12 weeks?

Antiphospholipid antibodies can be temporary. Repeating the test after at least 12 weeks shows whether the antibodies persist, which is what matters for lab criteria.

What do MPL units mean?

MPL stands for IgM phospholipid units, a scale many labs use for IgM anticardiolipin results. Labs set their own cutoffs, and other methods use different scales, so use the range printed on your own report.

Show the science and sources

For clinicians and researchers

Anticardiolipin reactivity in standard solid-phase assays is thought to be mediated by anti-beta-2 glycoprotein I antibodies binding beta-2 glycoprotein I complexed with cardiolipin. ELISA results are semi-quantitative in MPL (IgM phospholipid) units; chemiluminescent assays report U/mL or CU on different scales (one published comparison used a manufacturer cutoff of 20 U/mL for chemiluminescence versus 20 units, validated as the 99th percentile, for ELISA). The 2006 revised Sapporo (Sydney) laboratory criterion for APS is aCL IgG and/or IgM at medium or high titer (above 40 GPL or MPL, or above the 99th percentile) on two or more occasions at least 12 weeks apart, together with a clinical criterion. The 2023 ACR/EULAR classification criteria require a positive antiphospholipid test as an entry criterion and score the ELISA domain by isotype and level, with moderate (40 to 79 units) and high (80 units or more) cutoffs defined for standardized ELISA; IgM scores only 1 point compared with at least 4 for IgG, so isolated IgM positivity cannot meet the laboratory domain alone. The authors and later commentary stress these are classification criteria for research, not rules for diagnosis, and the fixed cutoffs do not transfer directly to chemiluminescent assays. In the 2019 EULAR/ACR SLE criteria, aCL at medium or high titer is part of the antiphospholipid antibody domain. ARUP Laboratories reports 12 MPL or less as negative, 13 to 19 as indeterminate, 20 to 80 as low to moderately positive and 81 or more as high positive.

LOINC 3182-3 (primary), 8067-1. Specimen: serum.

Sources

  1. Cardiolipin Antibody, IgM (test 0050902), ARUP Laboratories
  2. International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS), Journal of Thrombosis and Haemostasis (2006)
  3. The 2023 ACR/EULAR Antiphospholipid Syndrome Classification Criteria, Arthritis & Rheumatology (2023)
  4. 2023 American College of Rheumatology/European Alliance of Associations for Rheumatology classification criteria for antiphospholipid syndrome: good for patients or good for papers?, Research and Practice in Thrombosis and Haemostasis (2025)
  5. Detection of moderate to high antiphospholipid antibodies by chemiluminescence meets 2023 APS classification criteria, Scientific Reports (2025)
  6. Diagnosis of Antiphospholipid Syndrome by Chemiluminescent or Enzyme-Linked Immunosorbent Assay - A Comparison Study and Comprehensive Literature Review, Clinical and Applied Thrombosis/Hemostasis (2025)
  7. 2019 European League Against Rheumatism/American College of Rheumatology Classification Criteria for Systemic Lupus Erythematosus, Arthritis & Rheumatology (2019)
  8. Blood Clotting Disorders: Antiphospholipid Syndrome (APS), National Heart, Lung, and Blood Institute (NIH) (2022)
  9. Risk of developing antiphospholipid antibodies following viral infection: a systematic review and meta-analysis, Lupus (2018)
  10. Positive antiphospholipid antibodies: observation or treatment?, Journal of Thrombosis and Thrombolysis (2023)
  11. Systemic Lupus Erythematosus (Lupus): Diagnosis, Treatment, and Steps to Take, National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIH) (2022)
  12. 2020 American College of Rheumatology Guideline for the Management of Reproductive Health in Rheumatic and Musculoskeletal Diseases, Arthritis & Rheumatology (2020)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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