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Phospholipids are fat-like molecules with a hydrophilic, or water-loving, phosphate head and a hydrophobic, or water-repellent, lipid tail. Their special structure allows phospholipids to join together in layers to form cell membranes, and they take part in important cellular processes such as regulation of blood clotting. 

Antiphospholipid antibodies attack these critical phospholipids, creating a risk for dangerous blood clots anywhere in the body. In this article we’ll focus on the anticardiolipin antibody, a kind of antiphospholipid antibody that is especially common in people with lupus.

What Is the Anticardiolipin Antibody?

The anticardiolipin antibody (also known as aCL) specifically targets proteins on the surface of cardiolipin — a natural phospholipid molecule in our cell membranes and platelets that plays an important role in blood clotting. (Source)

While blood clotting is essential to keep us from bleeding uncontrollably when we have minor scrapes and injuries, excessive clotting can cause dangerous blockages in blood vessels. This kind of clot can stop blood from flowing to vital organs and deprive them of oxygen. The anticardiolipin antibody causes a high risk of developing dangerous blood clots. 

A Sign of Antiphospholipid Syndrome

The anticardiolipin antibody is one of three main antiphospholipid antibodies that can contribute to a rare autoimmune disease called antiphospholipid syndrome (APS). This syndrome is characterized by recurrent vascular thromboses (clots in the veins and arteries) and unexplained miscarriages, along with persistent presence of a specific set of antiphospholipid antibodies.

Because blood clots are not uncommon in other illnesses, APS can be difficult to diagnose. When symptoms suggest a person may have APS, they usually get tested for both the anticardiolipin antibody and another antiphospholipid antibody called lupus anticoagulant (LA), as the two antibodies are often found together. (Source)

But testing positive doesn’t necessarily mean that a person has APS. The anticardiolipin antibody can also be indicative of other autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis. In fact, more than a third of all SLE patients have some form of antiphospholipid antibodies, including the anticardiolipin antibody. (Source)

Anticardiolipin Antibody Testing 

Testing a patient for the anticardiolipin antibody can reveal the reason why they have a history of blood clots and help inform their treatment plans going forward. 

Who Is at Risk?

Antiphospholipid antibodies are found in 1%–5% of the population, but tend to be more likely in patients who suffer strokes, heart attacks, and clots. The development of these autoantibodies may have a genetic component, as they seem to run in families. 

If a person has APS or SLE, it’s possible their first-degree relatives (parents, siblings, or children) may have the anticardiolipin antibody. But whether those with the antibody will develop clinical symptoms can depend on epigenetic variations, such as the individual’s environment and lifestyle. (Source, Source)

Diagnostic tests are most commonly ordered for patients under 50 years of age who have suffered a major unexplained blood clot, people with existing autoimmune disorders, and women who have had repeated miscarriages or complications with their pregnancies. 

Other than the three main diagnostic criteria above, patients can also present with symptoms including:

  • cardiac conditions such as heart valve disease
  • thickening of the heart valves
  • livedo reticularis, a type of skin discoloration
  • a deficiency of platelets, which causes tissue bleeding


Types of Anticardiolipin Antibodies

The anticardiolipin antibody exists in three different forms: IgG, IgM, and IgA. These variations are called isotypes and although they attach to the same proteins on the surface of cardiolipin, they all have different structures. IgG and IgM anticardiolipin antibodies are associated with the highest risk of experiencing a major blood clot. (Source)

What Do My Test Results Mean?

If APS is suspected, you may have two or more blood tests at least 12 weeks apart, to determine if the anticardiolipin antibody is consistently present. You will only be diagnosed with APS if all tests come back positive. Weakly positive or inconsistent positive results are normal in healthy people, and are therefore not enough to support a diagnosis of APS. (Source)

A test is considered positive if it detects:

  • aCL IgM levels higher than 40.0 MPL 
  • or aCL IgG levels higher than 40.0 GPL 

(1 unit represents 1 microgram of the anticardiolipin antibody. MPL refers to the IgM isotype and GPL refers to IgG.) (Source, Source)

A consistently positive test result means you may have APS or another autoimmune disorder.

False Positive Syphilis Results

People who test positive for the anticardiolipin antibody sometimes falsely test positive for syphilis. This pattern was first described in the 1950s when many patients who falsely tested positive for syphilis were found to have autoimmune disorders, typically SLE.

Researchers later found that autoimmune patients had anticardiolipin antibodies that reacted with phospholipid substrates in the syphilis assay. The discovery ultimately led to the identification of APS as a separate disease. (Source)

What Is the Treatment for Anticardiolipin Antibody?

The treatment for people who test positive for aCL will depend on the symptoms that arise. If a blood clotting pattern starts to emerge, or APS is diagnosed, long-term blood thinning medications are the main form of treatment. 

Common anticoagulants (blood thinners) include: 

  • vitamin K antagonists (VKA) such as warfarin
  • low molecular weight heparin (LMWH)

Anticoagulants are often combined with antiplatelet medications, such as low dose aspirin. They work together to reduce the risk of developing dangerous blood clots.

Regular checkups are needed to make sure that the dose you’re taking still allows for enough clotting to prevent internal bleeding. (Source)

Treatment During Pregnancy

For a woman with antiphospholipid antibodies, starting anticoagulant therapy early in pregnancy offers the best chance of a successful outcome. 

Heparin, particularly LMWH, is the chosen anticoagulant for pregnant women because it doesn’t cross the placenta, and therefore doesn’t affect the fetus. The combination of heparin and low dose aspirin is recommended.

After her baby is delivered, a woman can be switched back to warfarin. (Source)

Treatment for SLE

When aCL is associated with blood clotting incidents in SLE patients, physicians will also prescribe a long-term anticoagulant treatment plan. But since SLE patients are at risk of renal disease and kidney failure that can be aggravated by these medications, lower doses are often recommended. (Source)

The Bottom Line on the Anticardiolipin Antibody

It’s important to remember that while the anticardiolipin antibody poses an increased health risk, testing positive does not mean you will experience a dangerous blood clot. If you are diagnosed with APS, or if a blood clotting pattern is emerging, you may be started on long-term anticoagulant medication to prevent excessive clotting. If you become pregnant, special care will be taken to help ensure a successful outcome. 

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Medically Reviwed
Medically Reviewed by
Dr. Deb Matthew
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