Antiphospholipid syndrome (APS) is an autoimmune disorder in which the immune system produces antibodies that make blood clot too easily. These abnormal clots can form in veins or arteries anywhere in the body, leading to strokes, deep vein thrombosis, pregnancy complications, and organ damage. An estimated 34,000 people in the United States had APS in 2019, with women affected roughly four times more often than men.
How APS Causes Blood Clots
In a healthy body, a protein called beta-2 glycoprotein I (β2GPI) helps clean up dead and dying cells by binding to their outer membranes. In APS, the immune system produces antibodies that latch onto this protein and interfere with its normal housekeeping role. Dead cell debris accumulates, and the antibodies simultaneously trigger inflammation along the walls of blood vessels. This combination of delayed cleanup and vascular inflammation creates the conditions for clots to form where they normally wouldn’t.
The antibodies also directly activate cells lining the blood vessels, switching on inflammatory signaling pathways that make those cells “stickier” and more prone to attracting clot-forming components. Animal studies have shown that infusing these antibodies increases clot size in a dose-dependent way: more antibodies, bigger clots. This is why the number and type of antibodies a person carries matters for predicting how aggressive the disease may be.
Who Gets APS
APS can develop on its own (called primary APS) or alongside another autoimmune condition, most commonly lupus. Nearly half of people with lupus test positive for at least one type of antiphospholipid antibody, and about a quarter carry the lupus anticoagulant, the antibody most strongly linked to clotting risk. Among lupus patients who carry the lupus anticoagulant, the 20-year risk of a blood clot reaches 50%.
Population-level data from the U.S. put the annual incidence at roughly 2.7 new cases per 100,000 people, with prevalence around 10 per 100,000. Women are diagnosed at nearly five times the rate of men. Estimates vary internationally, with reported prevalence ranging from about 17 per 100,000 in parts of Italy to over 60 per 100,000 in South Korea, likely reflecting differences in screening and diagnostic practices.
Clots, Strokes, and Other Complications
The hallmark of APS is abnormal clotting in vessels of all sizes. In one large cohort study of 361 APS patients, 36% experienced venous clots (typically deep vein thrombosis or pulmonary embolism), while about 25% had arterial clots outside the brain. Stroke is one of the most serious complications, affecting roughly one in four patients in the same study. Ischemic stroke, caused by a clot blocking blood flow to the brain, accounted for the vast majority of these events.
Even with treatment, nearly 12% of APS patients who suffered a stroke experienced a recurrence over a median follow-up of three years. APS is a particularly important diagnosis to consider in younger adults who have strokes without traditional risk factors like high blood pressure or high cholesterol.
Pregnancy Risks
APS is one of the most treatable causes of recurrent pregnancy loss, which makes diagnosis critical. The antibodies interfere with placental function, restricting blood flow to the developing fetus. Women with APS face roughly double the risk of fetal loss compared to women without the antibodies. Premature delivery is also more common.
The risk scales with how many different antibody types are present. In one study, women who tested positive for multiple types of antiphospholipid antibodies had a fetal loss rate of 48%, compared to 23% among those positive for only one type. Premature delivery followed a similar pattern: 57% in the multiple-positive group versus 17% in the single-positive group. With appropriate treatment during pregnancy, outcomes improve significantly, which is why testing for APS is standard practice after recurrent miscarriages.
How APS Is Diagnosed
Diagnosis requires both a clinical event (a confirmed blood clot or specific pregnancy complication) and laboratory evidence of antiphospholipid antibodies. The three antibody tests used are the lupus anticoagulant, anticardiolipin antibodies, and anti-beta-2 glycoprotein I antibodies. A single positive test isn’t enough. The 2023 classification criteria from the American College of Rheumatology and EULAR require that antibody positivity be confirmed on at least two separate blood draws spaced at least 12 weeks apart, within three years of the clinical event.
This repeat testing matters because infections, medications, and other temporary conditions can cause a one-time false positive. Persistent positivity is the key finding. The current classification system uses a weighted scoring approach across six clinical categories, including large-vessel clots, microvascular disease, obstetric complications, and heart valve abnormalities. Each category carries a point value, and a threshold score combined with confirmed antibody positivity establishes the diagnosis.
What “Triple Positive” Means
People who test positive for all three antibody types are sometimes called “triple positive,” and this pattern is generally considered the highest-risk profile. However, the relationship between antibody count and actual clotting events is not as straightforward as it might seem. One study found that roughly 43% of triple-positive individuals were asymptomatic “carriers” who had never had a clot. Among symptomatic patients, the number of positive antibody tests didn’t reliably predict recurrence or relapse. The strongest predictors of future clots in asymptomatic people appear to be two positive lupus anticoagulant tests combined with both anticardiolipin and anti-beta-2 glycoprotein I positivity, or persistent single anticardiolipin positivity without another autoimmune condition present.
Treatment and Management
The cornerstone of APS treatment is blood thinning medication to prevent new clots. For most patients with confirmed clotting events, this means long-term anticoagulation with a vitamin K antagonist, which requires regular blood monitoring to ensure the medication keeps clotting time in the right range. The target is typically an INR (a measure of how long blood takes to clot) between 2.0 and 3.0, though some patients with arterial clots may need a higher target.
Newer oral blood thinners that don’t require regular monitoring have been studied in APS, but clinical trials raised safety concerns. Several trials were stopped early because patients on newer agents had more clotting events than those on traditional therapy, particularly patients who were triple positive. As a result, current guidelines favor vitamin K antagonists for most APS patients with a history of clots.
For pregnant women with APS, treatment typically involves daily low-dose aspirin combined with injectable blood thinners. This combination has dramatically improved pregnancy outcomes, with live birth rates increasing from around 50% without treatment to over 70% with appropriate management.
Catastrophic Antiphospholipid Syndrome
In fewer than 1% of APS cases, the disease can escalate rapidly into a life-threatening condition called catastrophic APS (CAPS). Instead of clots forming in one or two locations, small clots develop simultaneously across multiple organs, typically affecting the kidneys, lungs, brain, and heart over a period of days. The mortality rate ranges from 36% to 48%, even with aggressive treatment.
CAPS is usually triggered by a specific event. The most common triggers include infections, surgery, trauma, and stopping anticoagulation medication. Pregnancy, cancer, estrogen-containing medications, and active lupus flares can also precipitate it. For people living with APS, understanding these triggers is important. Anticoagulation should never be stopped abruptly without medical guidance, and any planned surgery should involve careful coordination with the treating team to manage clotting risk during and after the procedure.