Antiphospholipid syndrome (APS) is an autoimmune disorder in which the immune system produces abnormal antibodies that make blood far more likely to clot. It affects roughly 40 to 50 people per 100,000 and is the most common acquired cause of excessive blood clotting, leading to deep vein thrombosis, stroke, heart attack, and recurrent pregnancy loss. APS can occur on its own (called primary APS) or alongside another autoimmune disease, most often lupus.
How APS Causes Blood Clots
In a healthy body, blood clots form only when you’re injured and dissolve once healing begins. In APS, three types of rogue antibodies disrupt that balance: lupus anticoagulant, anti-cardiolipin antibodies, and anti-beta-2-glycoprotein 1 antibodies. Despite the confusing name, lupus anticoagulant actually promotes clotting rather than preventing it. Having all three antibodies present at once, called “triple positivity,” carries the highest risk of serious clotting events.
These antibodies cause damage through several overlapping pathways. They irritate the cells lining blood vessels, reducing the availability of nitric oxide (a molecule that normally keeps vessels relaxed and open) while boosting production of compounds that constrict them. They also prime platelets, the small blood cells responsible for clotting, to overreact to even minor triggers like a small area of vessel damage. On top of that, the antibodies activate immune cells called neutrophils, which release sticky, web-like structures that trap blood cells and accelerate clot formation.
The clots themselves are physically different from normal ones. In APS, the fibrin mesh that forms a clot is denser, with thinner and more tightly branched fibers. This makes clots harder for the body to break down naturally, which is a key reason why clots in APS tend to persist and cause lasting damage.
Where Clots Form and What That Feels Like
APS clots can occur in veins, arteries, or the tiny vessels supplying organs and skin. The most common venous events are deep vein thrombosis (usually in the legs) and pulmonary embolism, where a clot travels to the lungs. Symptoms include leg swelling, pain, warmth, and sudden shortness of breath.
Arterial clots tend to hit the brain. In a large study of 1,000 APS patients, 20% developed a stroke over the course of follow-up, and 7% experienced transient ischemic attacks (mini-strokes). APS patients also face a higher risk of heart attack, with antiphospholipid antibodies found in about 11% of heart attack patients overall. Clots can also form in less typical locations like the kidneys or peripheral arteries.
Small vessel involvement produces subtler but still significant problems. Tiny clots near the skin surface can cause a lace-like purplish discoloration called livedo reticularis, digital gangrene, or slow-healing skin ulcers. In the brain, small vessel disease shows up as white matter changes on MRI scans, which are linked to cognitive difficulties, memory problems, and a higher long-term risk of dementia.
Pregnancy Complications
APS is one of the most important treatable causes of recurrent pregnancy loss. Fetal loss after 10 weeks of gestation is more strongly associated with the syndrome than earlier miscarriages, though roughly half of APS-related pregnancy losses still occur in the first trimester. A formal connection to APS is typically considered when someone has had three or more consecutive losses with no other identifiable cause.
Beyond miscarriage, the antibodies can impair blood flow through the placenta, leading to preeclampsia, a dangerous rise in blood pressure during pregnancy. Premature birth is estimated to occur in 30% to 60% of affected pregnancies, often driven by preeclampsia or restricted fetal growth. HELLP syndrome, a severe complication involving the breakdown of red blood cells, elevated liver enzymes, and low platelet counts, can also develop.
How APS Is Diagnosed
There is no single test that confirms APS. Diagnosis relies on a combination of clinical events (a confirmed blood clot or specific pregnancy complication) plus laboratory evidence of the antibodies. Critically, the antibodies must be found on at least two separate blood draws spaced weeks apart, because temporary antibody spikes can happen during infections or with certain medications without indicating true APS.
Updated classification criteria published jointly by the American College of Rheumatology and the European Alliance of Associations for Rheumatology in 2023 use a points-based system across six clinical domains and two laboratory domains. A minimum number of points in both categories is required. These criteria were designed primarily for research, but doctors use them in clinical practice as well. Testing is most commonly ordered for people who have unexplained blood clots (especially in unusual locations or at a young age), recurrent pregnancy loss, or clotting events that keep happening despite blood thinners.
Conditions That Can Look Like APS
Several other conditions cause clotting, low platelets, or pregnancy complications in ways that overlap with APS. Lupus is the most common mimic, since it shares vascular, neurological, and obstetric features, and frequently coexists with APS. Doctors distinguish the two by testing for lupus-specific antibodies and measuring complement levels in the blood.
Other conditions on the differential include heparin-induced thrombocytopenia (clotting triggered by heparin exposure), disseminated intravascular coagulation (widespread simultaneous clotting and bleeding, usually triggered by severe infection or surgery), and Behçet’s syndrome, a type of vasculitis that causes recurrent mouth and genital sores along with blood clots. A condition called Sneddon’s syndrome produces strokes and the same lace-like skin discoloration as APS but without the antibodies. Neurological symptoms of APS can even be mistaken for multiple sclerosis, since both cause brain lesions visible on MRI, though the lesion patterns differ.
Treatment: Blood Thinners as the Foundation
The backbone of APS treatment is long-term anticoagulation, most often with warfarin (a vitamin K antagonist). For a first venous clot, the standard target is an INR of 2 to 3, a measure of how effectively the drug is thinning the blood. For arterial clots like stroke, doctors may aim for a higher INR of 3 to 4, or combine standard-dose warfarin with low-dose aspirin, depending on the individual’s bleeding risk.
If clots recur despite adequate treatment, options include raising the INR target, adding aspirin, or switching to injectable low-molecular-weight heparin. For most patients, treatment is lifelong, since the underlying antibody production doesn’t go away and the risk of new clots remains high. In one study of triple-positive patients on standard warfarin therapy, 29% experienced a recurrent clot, underscoring that even treated APS requires close monitoring.
Why Newer Blood Thinners Aren’t Recommended
Direct oral anticoagulants (sometimes called DOACs), which are widely used for other clotting conditions and don’t require regular blood monitoring, have performed poorly in APS. A meta-analysis of three randomized trials found that patients on DOACs had four times the odds of recurrent clots compared to those on warfarin. Both the International Society on Thrombosis and Haemostasis and the American College of Chest Physicians now recommend against DOACs in APS, particularly for triple-positive patients. Warfarin remains the preferred choice despite the inconvenience of regular blood testing.
Living on Warfarin: Diet and Daily Life
Because warfarin works by blocking vitamin K, which the body uses to make clotting factors, your diet matters. The key isn’t to avoid vitamin K entirely but to keep your intake consistent from day to day so your medication dose stays effective. Sudden changes in how much vitamin K you eat can push your INR too high (raising bleeding risk) or too low (raising clotting risk).
High-vitamin-K foods to eat in consistent amounts include kale, spinach, broccoli, Brussels sprouts, collard greens, Swiss chard, mustard greens, and green vegetable juices. Lower-vitamin-K options that are less likely to affect your levels include carrots, corn, potatoes, sweet potatoes, tomatoes, mushrooms, green beans, cauliflower, and most fruits. You don’t need to memorize a list, but if you suddenly start or stop eating large servings of leafy greens, let your care team know so they can check your levels.
Regular INR monitoring, typically every few weeks once your dose is stable, is a non-negotiable part of managing APS on warfarin. Many patients use home testing kits that require just a finger prick, which makes the process significantly less disruptive.
Catastrophic APS: A Rare Emergency
Fewer than 1% of people with APS develop its most dangerous form, called catastrophic antiphospholipid syndrome (CAPS). In CAPS, clots form rapidly across multiple organs at once, causing sudden failure of the kidneys, lungs, heart, brain, or other systems over the course of days. Common triggers include infections, surgery, trauma, stopping anticoagulation, and sometimes cancer.
Even with aggressive treatment, CAPS carries a mortality rate between 37% and 48%. The best outcomes come from a combination approach using anticoagulation, high-dose steroids to suppress the immune response, and either plasma exchange or intravenous immunoglobulin. This triple therapy reduces mortality to roughly 29%, compared to 75% without treatment. Anyone with known APS who develops sudden multi-organ symptoms needs emergency care immediately.
Who Gets APS
APS affects women about twice as often as men. Roughly 85% of cases are primary, meaning they occur without another autoimmune disease. The remaining cases are most commonly associated with lupus, where antiphospholipid antibodies are found in a substantial proportion of patients. The estimated incidence is 1 to 2 new cases per 100,000 people per year, though the condition is likely underdiagnosed since many people don’t get tested until after a serious event like a stroke or multiple miscarriages.
APS can develop at any age but is most commonly diagnosed in young to middle-aged adults. Because it often strikes people who wouldn’t otherwise be expected to have strokes or blood clots, unexplained clotting in someone under 50 is one of the classic red flags that prompts testing.