What this test measures
Anticardiolipin IgM detects IgM-class autoantibodies against cardiolipin, a phospholipid in mitochondrial membranes and on activated platelets. IgM is usually the earlier antibody response, sometimes appearing before IgG in newly developed antiphospholipid syndrome.
The test is a quantitative ELISA reported in MPL units per mL. It is one of the three primary laboratory criteria of the 2023 ACR/EULAR APS classification, alongside lupus anticoagulant and anti-β2-glycoprotein-1 antibodies.
Why it matters
IgM aCL is part of the standard APS workup. Although less predictive of thrombosis than IgG, persistent medium-high IgM positivity still meets criteria and contributes to the diagnosis. Some patients are positive for IgM alone, especially early in disease or after infection.
In Indian clinical practice, aCL IgM is included in any APS workup — for unexplained thrombosis, recurrent miscarriage, lupus follow-up, or unusual-site clotting (cerebral venous sinus thrombosis, splanchnic thrombosis). The 2023 criteria require confirming persistence on a second test at ≥12 weeks because transient IgM positivity is well-described after viral infections.
How to prepare
No fasting required. Continue all medications, including anticoagulants — warfarin, DOACs, and heparin do not interfere with aCL antibody testing.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Anticardiolipin IgM (MPL U/mL)[1][2] | Negative: < 20 MPL · Medium-high titre: ≥ 40 MPL | Negative — does not rule out APS; need full panel. | Medium-high titre on two occasions ≥12 weeks apart is a primary APS lab criterion. IgM positivity has lower predictive value for thrombosis than IgG; clinical context essential. |
aCL IgM vs IgG — clinical predictive value
| Feature | IgM | IgG |
|---|---|---|
| Thrombosis risk | Moderate | High |
| Pregnancy loss risk | Moderate | High |
| Transient after infection | Common | Less common |
| Required confirmation | ≥12 weeks | ≥12 weeks |
Frequently asked questions
Do I need to fast for aCL IgM?
No fasting needed.
My IgM aCL is positive but IgG is negative — does that mean APS?
Not necessarily. Isolated IgM positivity requires confirmation at ≥12 weeks and clinical correlation. Transient IgM aCL after viral infections is well-described.
Why is IgM less predictive than IgG?
IgM antibodies are usually the early response and may be transient. IgG persistence is more strongly associated with established APS and clot risk.
Can anticoagulants affect the result?
No. Unlike lupus anticoagulant testing, anticardiolipin antibody assays are unaffected by warfarin, DOACs, or heparin.
Should I be tested in pregnancy?
Yes if you have unexplained recurrent miscarriage, severe pre-eclampsia, or known APS. Treatment with aspirin + low molecular weight heparin dramatically improves outcomes in APS pregnancies.
How long does the report take?
Typically 2–3 days.
Related Autoimmune / Rheumatology tests
Tests commonly ordered alongside CARDIOLIPIN ANTIBODY (ACL) - IgM, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — Antiphospholipid Antibodies · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Antiphospholipid Syndrome · accessed 2026-05-30T00:00:00.000Z
- ACR/EULAR 2023 APS Classification Criteria · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Anticardiolipin Antibodies · accessed 2026-05-30T00:00:00.000Z
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