Anticardiolipin antibodies test explained (aCL)
- Updated
- 2026-09-10
- Every number with its source
What anticardiolipin antibodies are
Anticardiolipin antibodies are a group of autoantibodies that the immune system mistakenly produces against cardiolipin — a fatty molecule found in cell membranes and in proteins involved in blood clotting. They belong to the broader group of antiphospholipid antibodies, together with lupus anticoagulant and anti-β2 glycoprotein I antibodies. These antibodies can disrupt the normal blood clotting process and increase the risk of a blocked blood vessel (thrombosis), and they are also linked with pregnancy complications — miscarriages, preterm birth, preeclampsia. When anticardiolipin antibodies (together with other antiphospholipid antibodies) are found repeatedly in two tests taken at least 12 weeks apart, together with characteristic clinical features (thrombosis or pregnancy losses), antiphospholipid syndrome is diagnosed. The exact mechanism of why the immune system starts producing these antibodies is not fully clear — viral or bacterial infections that mimic the body's own proteins, as well as genetic factors, are thought to play a part.
How the result is reported
The sources we use do not give a common numeric norm with units — the result is usually assessed as negative or positive (a low, medium or high titre), and the cut-off value depends on the particular laboratory's method. You will find your own laboratory's exact scale on your result sheet.
| What is assessed | What it means according to the sources | What still has to be taken into account |
|---|---|---|
| A negative or positive result | The sources we use do not give a common numeric norm with units — the result is usually assessed as negative or positive (a low, medium or high titre) | The cut-off value depends on the particular laboratory's method; you will find your own laboratory's exact scale on your result sheet |
| A raised (positive) result | According to MedlinePlus and the NHS, it is linked with antiphospholipid syndrome — an autoimmune condition in which the immune system mistakenly attacks proteins involved in clotting and increases the risk of blood clots and pregnancy complications | A single positive result does not confirm the diagnosis — a repeat test after several months is needed |
| A repeat test after at least 12 weeks | Antiphospholipid syndrome is diagnosed when the antibodies are found repeatedly in two tests taken at least 12 weeks apart, together with characteristic clinical features (thrombosis or pregnancy losses) | A temporary rise in the antibodies can follow an infection |
When the test is done
- When there has been a blood clot (deep vein thrombosis, pulmonary embolism, stroke), especially at a younger age or without a clear cause
- After repeated miscarriages (3 or more) or other pregnancy complications — preterm birth, preeclampsia, stillbirth
- When systemic lupus erythematosus or another autoimmune disease is suspected, with which antiphospholipid antibodies are often linked
- When laboratory tests show a prolonged blood clotting time (APTT)
- When unusual skin changes appear, for example a blue net-like pattern on the skin (livedo reticularis)
How to prepare
- No special preparation for the test is usually needed, and blood is taken from a vein (see also: how to prepare for a blood test)
- To confirm the diagnosis, a doctor usually asks for the test to be repeated after at least 12 weeks, because a temporary rise in the antibodies can follow an infection
A raised (positive) result
A raised (positive) anticardiolipin antibody result, according to MedlinePlus and the NHS, is linked with antiphospholipid syndrome — an autoimmune condition in which the immune system mistakenly attacks proteins involved in clotting and increases the risk of blood clots and pregnancy complications. A single positive result does not confirm the diagnosis — a repeat test after several months is needed.
Possible causes of a raised result
- Antiphospholipid syndrome — primary or secondary (alongside another autoimmune disease)
- Systemic lupus erythematosus and other autoimmune diseases
- Past or current viral or bacterial infections, which can temporarily trigger the production of these antibodies
- Some medicines, which in rare cases temporarily cause a similar rise in the antibodies
Symptoms that can accompany a raised result
- Blood clots in veins or arteries — deep vein thrombosis, pulmonary embolism, stroke
- Repeated miscarriages or other pregnancy complications
- A reduced platelet count
- A blue net-like pattern on the skin (livedo reticularis)
- Skin ulcers
- Migraine
What to discuss with a doctor
- A repeat anticardiolipin antibody test after at least 12 weeks, to confirm the diagnosis
- The other antiphospholipid tests — lupus anticoagulant, anti-β2 glycoprotein I antibodies
- The clinical history — whether there have been thromboses or pregnancy losses
- A rheumatologist or haematologist consultation for further assessment and monitoring
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Frequently asked questions
›What is the normal range for anticardiolipin antibodies?
The sources we use do not give a common numeric norm with units — the result is usually assessed as negative or positive (a low, medium or high titre), and the cut-off value depends on the particular laboratory's method. You will find your own laboratory's exact scale on your result sheet.
›What does a positive anticardiolipin antibody result mean?
According to MedlinePlus, a positive result shows antiphospholipid antibodies in the blood, which are linked with an increased risk of blood clots and pregnancy complications. A single positive result does not confirm the diagnosis — diagnosing antiphospholipid syndrome requires a repeat positive test after at least 12 weeks, together with clinical features.
›Why is this test ordered together with the other antiphospholipid tests?
Anticardiolipin antibodies are only one of the three main groups of antiphospholipid antibodies — lupus anticoagulant and anti-β2 glycoprotein I antibodies are tested alongside them. The results of all three tests together help to assess the likelihood of antiphospholipid syndrome more precisely.
›Does a positive result always mean antiphospholipid syndrome?
No. The antibodies can rise temporarily after an infection or while taking certain medicines, and in some people the result can also be positive without any disease. A diagnosis requires a repeat positive test after at least 12 weeks, together with characteristic clinical features — thromboses or pregnancy losses.
›When does a doctor order this test?
The test is usually ordered after an unexpected blood clot, especially at a younger age or without a clear cause, or after repeated miscarriages and other pregnancy complications. It may also be ordered when systemic lupus erythematosus or another autoimmune disease is suspected.
›Do I need to prepare specially for the test?
No, no special preparation is usually needed — blood is taken from a vein in the usual way. To clarify the diagnosis, a doctor may ask for the test to be repeated after a few months.
Sources
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.