Blood clots are semi-solid masses of blood that form when proteins and platelets in your blood stick together. They’re a normal part of healing: when you cut yourself, a clot seals the wound and stops the bleeding. But when clots form inside your veins or arteries without an injury, or when they don’t dissolve on their own, they become dangerous. Up to 900,000 people in the United States are affected by blood clots each year, and an estimated 60,000 to 100,000 die from them.
How Blood Clots Form
Clot formation starts the moment a blood vessel is damaged. Cells in the injured tissue release a chemical signal, sometimes called tissue factor, that acts like a switch turning on the clotting process. This triggers a chain reaction involving more than a dozen specialized proteins in your blood, each activating the next in sequence.
The chain reaction ultimately produces a protein called thrombin. Thrombin converts a substance already floating in your blood (fibrinogen) into fibrin, a sticky, thread-like material that weaves itself into a mesh. That mesh traps platelets and red blood cells, forming a plug over the wound. A final protein in the chain then reinforces the mesh so the clot holds firm while the tissue underneath heals. Once healing is complete, your body gradually dissolves the clot.
Problems arise when this system misfires. A clot can form inside a blood vessel where there’s no open wound, or a clot that forms normally can break loose and travel to a dangerous location.
Arterial Clots vs. Venous Clots
Not all blood clots are the same. Arterial clots form in the arteries that carry oxygen-rich blood away from the heart. They tend to be packed with platelets and typically develop around fatty buildup on artery walls. When an arterial clot blocks blood flow to the heart, it causes a heart attack. When it blocks flow to the brain, it causes a stroke.
Venous clots form in the veins that return blood to the heart. These clots contain more fibrin than platelets and are more likely to develop in areas of slow-moving blood, particularly the deep veins of the legs. This distinction matters for treatment: arterial clots are generally treated with drugs that target platelets, while venous clots are treated with anticoagulants that interrupt the clotting chain reaction.
Deep Vein Thrombosis and Pulmonary Embolism
The most common dangerous venous clot is a deep vein thrombosis, or DVT, which forms in the large veins deep inside the leg. A DVT can partially or fully block blood flow in the vein, causing swelling, pain, and skin discoloration. But the real danger is what happens if the clot breaks free. It can travel through the bloodstream, pass through the right side of the heart, and lodge in an artery in the lung. This is called a pulmonary embolism, or PE.
A pulmonary embolism blocks blood flow to part of the lung, which can be fatal. In about 25% of PE cases, sudden death is the first symptom, meaning there’s no warning before the clot reaches the lung. Together, DVT and PE are referred to as venous thromboembolism, or VTE.
Symptoms to Recognize
A blood clot in the leg often causes swelling, pain or cramping (usually starting in the calf), a feeling of warmth in the affected area, and a change in skin color to red or purple. These symptoms typically appear in one leg, not both. However, DVT can also occur without any noticeable symptoms at all, which is part of what makes it so dangerous.
A pulmonary embolism may cause sudden shortness of breath, chest pain that worsens with deep breathing, a rapid heartbeat, coughing (sometimes with blood), and lightheadedness or fainting. These symptoms develop quickly and require emergency medical attention.
Who Is at Higher Risk
Several factors increase your risk of developing a blood clot. The CDC groups them into three main categories: vein injury, slow blood flow, and increased estrogen.
- Vein injury from major surgery, especially procedures involving the abdomen, pelvis, hip, or legs, can trigger clotting at the surgical site.
- Slow blood flow is caused by extended bed rest, hospitalization, wearing a cast, or sitting for long periods with crossed legs. Any situation that keeps blood pooling in the legs raises your risk.
- Increased estrogen from estrogen-containing birth control, hormone replacement therapy after menopause, or pregnancy (with risk continuing up to three months after giving birth) makes blood more prone to clotting.
Inherited clotting disorders also play a role. Some people are born with genetic variations that make their blood clot more easily than normal. If you have a family history of blood clots, your own risk is higher.
How Blood Clots Are Diagnosed
Diagnosis usually starts with a blood test called a D-dimer. This test measures a substance released when a clot breaks down. A negative D-dimer result generally means you don’t have a clot, making it a useful screening tool. A positive result, however, doesn’t confirm a clot on its own, since D-dimer levels can rise for other reasons like infection or recent surgery.
If the D-dimer is positive, the next step is typically a duplex ultrasound. This imaging test uses sound waves to visualize blood flow in your veins and can detect blockages or clots in the deep veins. It’s the standard imaging test for diagnosing DVT: painless, noninvasive, and usually done right in the clinic or hospital. For suspected pulmonary embolism, a CT scan of the chest with contrast dye is the most common diagnostic tool.
Treatment Options
The primary treatment for blood clots is anticoagulant medication, commonly called blood thinners. These drugs don’t actually thin your blood. Instead, they interrupt the clotting chain reaction to prevent existing clots from growing and new ones from forming. Your body then gradually dissolves the clot on its own.
Several types of anticoagulants work in different ways. Heparin, often used in the hospital, activates a natural anti-clotting protein already in your blood. Warfarin blocks your body’s use of vitamin K, which is a key ingredient in the clotting process. Newer oral anticoagulants block specific proteins in the clotting chain and have become widely used because they require less monitoring than warfarin and fewer dietary restrictions.
Treatment length varies. For a first-time clot with a clear trigger like surgery, you may take anticoagulants for three to six months. If you have a clotting disorder or a clot with no identifiable cause, your doctor may recommend longer or even indefinite treatment. In life-threatening cases like a massive pulmonary embolism, more aggressive clot-dissolving medications can be used to break down the clot rapidly.
Preventing Blood Clots
Prevention focuses on keeping blood moving, especially during the situations that put you at highest risk. After surgery, hospitals routinely use compression devices on your legs and encourage you to get up and walk as soon as safely possible. Graduated compression stockings, available at pharmacies and surgical supply stores, help prevent blood from pooling in your lower legs.
During long flights or car rides, stand up and stretch as frequently as possible. Walk the aisle when you can, and stay hydrated with water or other nonalcoholic beverages, since dehydration thickens the blood and contributes to clot formation. Even simple movements like flexing your ankles and contracting your calf muscles while seated can help keep blood circulating.
If you’re on estrogen-containing birth control and have other risk factors, like a family history of clotting or an upcoming surgery, talk to your provider about whether your current contraception is the right choice. Small adjustments to medications and habits can meaningfully reduce your risk.