What this test measures
Anti-phospholipid antibody IgM (often anticardiolipin IgM by ELISA). One of the laboratory criteria for APS but less specific and more often transient than IgG. Like IgG, must persist for at least 12 weeks at moderate-to-high titre to satisfy classification criteria. Combined IgG + IgM testing is standard; IgG alone is more specific for clinically meaningful APS.
Why it matters
Same clinical indications as APL IgG — unexplained thrombosis in young, recurrent miscarriage, late fetal loss, severe preeclampsia, SLE workup. IgM positive alone (without IgG, LA, or anti-β2GPI) is less strongly associated with APS clinical events. Transient IgM positivity is common after viral infections (EBV, HIV, hepatitis C, malaria, COVID-19), drugs (chlorpromazine, hydralazine, procainamide, some antibiotics), and bacterial infections — repeat in 12 weeks is essential.
How to prepare
No fasting required. Sample BEFORE starting anticoagulation if possible. Disclose pregnancy, recent infections, current medications, immunosuppression.
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 |
|---|---|---|---|
| Anti-Phospholipid IgM (MPL units (or U/mL))[1] | < 20 MPL (low); 20–40 (moderate); > 40 (high) | Negative or low (< 20 MPL): no APS by this criterion. | Moderate (20–40) or High (> 40) — repeat at 12 weeks to confirm persistence. Single positive can be transient. Strong association with APS requires persistent moderate–high titre + clinical features. Order IgG, anti-β2-glycoprotein-I, and lupus anticoagulant for the full APS panel. |
APS antibody criteria summary
| Test | Criterion threshold | Specificity |
|---|---|---|
| Anticardiolipin IgG | > 40 GPL persisting 12 weeks | High |
| Anticardiolipin IgM | > 40 MPL persisting 12 weeks | Moderate |
| Anti-β2-GPI IgG | > 40 U persisting 12 weeks | High |
| Anti-β2-GPI IgM | > 40 U persisting 12 weeks | Moderate |
| Lupus anticoagulant | Positive persisting 12 weeks | Highest (functional test) |
Frequently asked questions
Why is IgM less specific than IgG?
IgM is the first antibody produced after exposure (immature response) and is often transient after infections. IgG is a more mature response and more strongly associated with persistent APS pathology.
Can a recent COVID-19 infection cause positive IgM?
Yes — viral infections including COVID, HIV, EBV, hepatitis C, and bacterial infections can cause transient anti-phospholipid IgM positivity. Repeat at 12 weeks to confirm.
My IgM is positive but IgG is negative — should I worry?
Isolated IgM positivity has weaker association with APS clinical events than IgG. Watch for clinical features and repeat in 12 weeks. Treatment decisions need persistence + clinical context.
Should I get the full APS panel?
Yes — if APS is suspected (unexplained thrombosis or pregnancy complications), order all three antibody types (anticardiolipin, anti-β2GPI, lupus anticoagulant) — both IgG and IgM. Triple positivity is the strongest risk pattern.
Do I need to stop blood thinners before the test?
Anticardiolipin and anti-β2GPI ELISAs are NOT affected by anticoagulation. Lupus anticoagulant (a clotting-based test) IS affected — vitamin K antagonist users should ideally be tested before starting or after bridging. Discuss with your doctor.
Related Lipids / Cardiac Risk tests
Tests commonly ordered alongside ANTI PHOSPHOLIPID ANTIBODY (APL) - IGM, or that help interpret an unexpected result.
Sources & references
- Updated APS Criteria (2023 ACR/EULAR) · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Antiphospholipid Antibodies · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Antiphospholipid Syndrome · accessed 2026-05-30T00:00:00.000Z
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