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. The anticardiolipin antibody can be classified as an autoantibody, as aCL targets and attacks your own cells. (Source)
While blood clotting is essential to keep you from bleeding uncontrollably when you have minor scrapes and injuries, excessive clotting (thrombosis) 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. (Source)
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. (Source, Source)
Because blood clots are not uncommon in some 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 blood clots. The development of these autoantibodies may have a genetic component, as like many autoimmune conditions they seem to run in families. (Source)
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 — that is, influences on how and whether genes are turned on or off — 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
Anticardiolipin Antibodies and COVID
While studies on the association between COVID and autoimmune conditions are still relatively new, research has demonstrated a link between anticardiolipin antibodies and patients with severe forms of COVID. Both are indicative of inflammation and could lead to worsening outcomes, such as thrombosis. Given the possible connection between COVID and lupus, as well as aCL and lupus, this is an area that warrants further research and caution from patients. (Source)
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 2 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 (i.e., 40 mcg IgM), or
- aCL IgG levels higher than 40.0 GPL (i.e., 40 mcg IgG)
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 with falsely positive syphilis tests were found to have autoimmune disorders, typically lupus.
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)