Every year, blood clots — specifically venous thromboembolism (VTE) — affect an estimated 900,000 Americans and kill approximately 100,000, making this condition a leading cause of preventable hospital death. Yet public awareness remains alarmingly low. A survey by the American Blood Clot Association found that fewer than 25 percent of Americans could identify the major signs of a blood clot. These are not obscure medical events — they can happen to anyone, at any age, and recognizing the symptoms quickly can be the difference between a treatable condition and a fatal one. For context on how vascular conditions connect to overall health, visit our medical conditions guide.
What Are Blood Clots?
Blood clotting (coagulation) is a normal, life-saving process. When you cut yourself, platelets and clotting proteins rapidly form a clot to stop bleeding. Problems arise when clots form inside blood vessels where they aren’t needed or when they fail to dissolve after serving their purpose.
The two major types of pathologic blood clots are arterial clots and venous clots. Arterial clots typically form in arteries narrowed by atherosclerosis and are the cause of most heart attacks and strokes. Venous clots — the focus of this guide — form in veins, usually the deep veins of the legs or pelvis. According to the CDC, venous thromboembolism encompasses two related conditions: deep vein thrombosis (DVT), a clot in a deep vein, and pulmonary embolism (PE), which occurs when a DVT fragment breaks free and travels to the lungs.
Superficial thrombophlebitis — clotting in a vein near the skin’s surface — is generally less dangerous than DVT, causing localized pain, redness, and swelling along the affected vein. However, superficial clots near the junction with the deep venous system can extend and become a DVT, so they shouldn’t be entirely dismissed.
Causes and Risk Factors
In 1856, German pathologist Rudolf Virchow identified three broad categories of factors that predispose to clot formation — collectively known as Virchow’s triad: stasis (slowed blood flow), endothelial injury (damage to the blood vessel wall), and hypercoagulability (increased tendency of blood to clot). Most blood clots result from a combination of these factors.
Common situational risk factors include surgery (especially orthopedic procedures — hip and knee replacement carry among the highest risks), hospitalization and immobility, long-distance travel (flights or drives lasting more than 4 hours), pregnancy and the postpartum period, and the use of estrogen-containing medications (oral contraceptives and hormone replacement therapy). According to the NHLBI, surgery and hospitalization account for approximately 60 percent of all VTE events.
Medical risk factors include cancer (which increases VTE risk four to sevenfold), prior VTE (recurrence rate is approximately 30 percent over 10 years without ongoing anticoagulation for unprovoked events), obesity, heart failure, inflammatory bowel disease, nephrotic syndrome, and COVID-19 infection (which is associated with a prothrombotic state). Age is a major factor — VTE risk approximately doubles with each decade after age 40.
Inherited thrombophilias — genetic conditions that increase clotting tendency — include Factor V Leiden mutation (present in about 5 percent of Caucasians), prothrombin G20210A mutation, protein C deficiency, protein S deficiency, and antithrombin deficiency. Acquired thrombophilia, particularly antiphospholipid syndrome, also significantly increases VTE risk.
Symptoms of Deep Vein Thrombosis (DVT)
DVT most commonly occurs in the legs, though it can develop in the arms (upper extremity DVT, often associated with central venous catheters or thoracic outlet syndrome) or other veins. According to the Mayo Clinic, symptoms of leg DVT include swelling in one leg (asymmetric leg swelling is a key finding), pain or tenderness in the leg (often starting in the calf and described as a cramp or soreness), redness or discoloration of the skin, and warmth over the affected area.
Importantly, approximately half of DVTs produce no noticeable symptoms, making them difficult to detect without imaging. When symptoms are present, they typically affect only one leg. Bilateral symptoms are less common and may suggest an alternative diagnosis or a large clot blocking the iliac veins or inferior vena cava.
Post-thrombotic syndrome (PTS) is a long-term complication of DVT that develops in 20 to 50 percent of patients. It results from damage to the venous valves caused by the clot, leading to chronic venous insufficiency. Symptoms include persistent leg swelling, pain, heaviness, skin discoloration (particularly brown pigmentation around the ankles), and in severe cases, venous ulcers.
Symptoms of Pulmonary Embolism (PE)
Pulmonary embolism occurs when a clot — usually originating from a leg DVT — travels through the bloodstream and lodges in a pulmonary artery, blocking blood flow to a portion of the lung. PE is a medical emergency. The CDC estimates that sudden death is the first symptom in about 25 percent of people with PE.
Symptoms of PE include sudden shortness of breath (the most common symptom), sharp chest pain that worsens with deep breathing or coughing (pleuritic chest pain), rapid heart rate (tachycardia), cough (sometimes with blood-tinged sputum), lightheadedness or fainting, and anxiety or a sense of impending doom. Massive PE can cause cardiovascular collapse, severe hypotension, and loss of consciousness.
When to seek emergency care: Call 911 immediately if you experience sudden shortness of breath, chest pain that worsens with breathing, coughing up blood, rapid heartbeat with lightheadedness or fainting, or if you have leg swelling with any of these respiratory symptoms. Pulmonary embolism is fatal in up to 30 percent of untreated cases but is highly treatable when diagnosed promptly.
Diagnosis
Diagnosis of DVT begins with clinical assessment using validated scoring systems — the Wells score for DVT assigns points based on clinical features to stratify patients into low, moderate, and high probability categories. D-dimer, a blood test that measures a fibrin degradation product, is useful as a “rule-out” test: a negative D-dimer in a low-probability patient effectively excludes DVT. However, D-dimer is elevated in many conditions (infection, inflammation, pregnancy, cancer, recent surgery) and is therefore not specific for clots.
Compression ultrasonography is the primary imaging test for DVT. It is non-invasive, widely available, and highly accurate for proximal DVT (above the knee), with sensitivity and specificity both above 95 percent. For suspected calf vein DVT or when ultrasound is inconclusive, CT venography or MR venography may be used.
For suspected PE, CT pulmonary angiography (CTPA) is the gold standard imaging test, with sensitivity exceeding 95 percent. According to the Radiological Society of North America, CTPA provides rapid, detailed visualization of the pulmonary arteries and can also reveal alternative diagnoses. Ventilation-perfusion (V/Q) scanning is an alternative for patients who cannot receive CT contrast (e.g., severe contrast allergy or kidney impairment). Echocardiography can detect right ventricular strain from massive PE and help guide the urgency of treatment.
Treatment
Anticoagulation: The Foundation of Treatment
Anticoagulation (“blood thinners”) is the mainstay of VTE treatment. These medications don’t dissolve existing clots but prevent clot extension and new clot formation, allowing the body’s natural fibrinolytic system to gradually break down the thrombus. According to the American Society of Hematology, direct oral anticoagulants (DOACs) — including rivaroxaban (Xarelto), apixaban (Eliquis), edoxaban (Savaysa), and dabigatran (Pradaxa) — are now preferred over warfarin (Coumadin) for most patients with VTE.
DOACs offer several advantages: fixed dosing without routine blood monitoring, fewer drug and food interactions, rapid onset of action, and at least equivalent efficacy with potentially lower bleeding risk. Rivaroxaban and apixaban can be used as single-drug therapy from the start. Edoxaban and dabigatran require initial treatment with a parenteral anticoagulant (typically low-molecular-weight heparin) for 5 to 10 days.
Warfarin remains appropriate in certain situations, including patients with mechanical heart valves, antiphospholipid syndrome, severe kidney impairment, or those who cannot afford DOACs. When warfarin is used, it is bridged with heparin until the INR is therapeutic (2.0 to 3.0) for at least 24 hours.
Duration of Anticoagulation
Treatment duration depends on whether the VTE was provoked (triggered by a temporary risk factor) or unprovoked. For VTE provoked by a major transient risk factor (surgery, major trauma), 3 months of anticoagulation is standard. For unprovoked VTE or VTE associated with a persistent risk factor (cancer, ongoing immobility), extended or indefinite anticoagulation is typically recommended, as the recurrence risk without ongoing treatment is substantial — approximately 10 percent in the first year and 30 percent over 10 years.
Advanced Treatments
For massive PE with hemodynamic instability (low blood pressure, shock), systemic thrombolysis (clot-dissolving medication, typically alteplase) can be lifesaving. Catheter-directed therapy — delivering thrombolytic drugs directly to the clot or mechanically fragmenting it — is increasingly used for submassive PE (PE with right ventricular strain but stable blood pressure). Surgical embolectomy is reserved for the most critical cases when thrombolysis is contraindicated or has failed.
Inferior vena cava (IVC) filters — small devices placed in the IVC to catch traveling clots — are used in patients with VTE who cannot take anticoagulation (e.g., active bleeding, upcoming surgery). Retrievable IVC filters should be removed once the contraindication to anticoagulation resolves, as leaving them in place long-term carries risks of IVC thrombosis and filter migration.
Prevention
VTE prevention (thromboprophylaxis) is one of the most important patient safety measures in hospitalized patients. The American College of Chest Physicians recommends pharmacologic prophylaxis (low-molecular-weight heparin or low-dose unfractionated heparin) for most hospitalized medical and surgical patients at increased VTE risk. Mechanical prophylaxis with intermittent pneumatic compression devices is used when pharmacologic prophylaxis is contraindicated.
After orthopedic surgery — particularly total hip and knee replacement — extended prophylaxis for up to 35 days is recommended due to the prolonged high-risk period. For cancer patients undergoing surgery, extended prophylaxis is also standard. Cancer patients receiving chemotherapy as outpatients may benefit from thromboprophylaxis with DOACs in select high-risk cases.
For healthy individuals, common-sense prevention during high-risk situations includes staying active during long flights or car trips (walk the aisle, do calf exercises, stay hydrated), discussing VTE risk with your doctor before surgery, maintaining a healthy weight, staying physically active, and understanding that estrogen-containing medications increase risk. Knowing your family history of blood clots can also inform risk-reduction strategies. Understanding the cost of prevention versus treatment underscores why proactive measures are worthwhile.
Frequently Asked Questions
Can you feel a blood clot forming?
Not always. About half of DVTs produce no symptoms. When symptoms do occur, they typically develop gradually over hours to days — a progressive ache, swelling, or warmth in one leg. You won’t necessarily feel the clot “forming” as a discrete event. What you may notice is a change: one leg becomes more swollen, painful, or warm than the other. Any unexplained unilateral leg swelling, especially if accompanied by pain, should prompt medical evaluation.
How serious is a blood clot in the leg?
A DVT is a serious condition that requires treatment. The immediate danger is that a piece of the clot can break off and travel to the lungs, causing a pulmonary embolism — which can be fatal. With prompt anticoagulation treatment, the risk of PE is greatly reduced, and most patients recover fully. The long-term concern is post-thrombotic syndrome, which develops in 20 to 50 percent of DVT patients and causes chronic leg symptoms. Early treatment and use of compression stockings can reduce this risk.
Can blood clots go away on their own?
Small superficial blood clots may resolve without treatment. However, DVTs and PEs should never be left untreated. While the body’s natural fibrinolytic system does work to dissolve clots over time, anticoagulation is essential to prevent clot extension, PE, and recurrence while dissolution occurs. Without treatment, the risk of fatal PE and long-term complications is unacceptably high. Most DVTs that are treated appropriately with anticoagulation show significant resolution within weeks to months.
Does aspirin prevent blood clots?
Aspirin is an antiplatelet agent that primarily prevents arterial clots (the type involved in heart attacks and strokes). It has limited effectiveness for venous blood clots (DVT/PE), which are driven more by the coagulation cascade than by platelet aggregation. Some studies suggest a modest benefit of aspirin for VTE prevention after orthopedic surgery or as extended prevention after completing initial anticoagulation for VTE. However, aspirin is not a substitute for proper anticoagulation to treat existing venous clots.
Can you fly with a blood clot?
If you have an acute DVT or PE, air travel should be avoided until you have been on stable anticoagulation for an adequate period — most experts recommend at least 2 to 4 weeks of treatment. Long flights can worsen venous stasis and increase the risk of clot extension. Once adequately anticoagulated, most patients with stable VTE can travel safely with precautions: wear compression stockings, stay hydrated, walk the aisle periodically, and take medication as prescribed. Consult your doctor before flying with a recent or current blood clot.
Recognizing the Signs Could Save Your Life
Blood clots are common, dangerous, and treatable — but the treatment window matters. If you develop sudden swelling in one leg, particularly with pain or warmth, see a doctor the same day. If you experience sudden shortness of breath, chest pain with breathing, or rapid heartbeat — call 911. These are not symptoms to monitor overnight or manage with home remedies.
Know your risk factors. If you’re having surgery, ask your surgeon about VTE prevention measures. If you’re taking estrogen-containing birth control or hormone replacement, understand that this modestly increases your VTE risk. If you have a family history of blood clots, particularly at a young age, discuss thrombophilia testing with your doctor. If you’ve had a VTE before, understand your recurrence risk and discuss the appropriate duration of anticoagulation.
The statistics are clear: tens of thousands of VTE deaths occur annually in the United States, and the majority are preventable with awareness, prevention, and prompt treatment. Knowledge of the symptoms and risk factors is the single most important tool you have — and it costs nothing.