Thrombosis is the medical term for when a blood clot, or “thrombus,” forms a blockage inside a blood vessel.

The thrombus limits or blocks blood flow to the parts of the body that the vessel usually supplies, causing symptoms in those areas.

Without treatment, some blood clots can cause serious complications and even death. Receiving treatment early reduces these risks and improves the person’s outlook.

This article looks further into what thrombosis is, including the types and their symptoms. It also explores the risk factors and causes of this issue and describes the diagnostic process, as well as treatment and prevention techniques.

A doctor checking a scan for signs of thrombosis.Share on Pinterest
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Blot clots are a normal biological response to an injury. They help seal a damaged blood vessel to prevent excessive bleeding. Blood clots typically break down and dissolve into the bloodstream over time.

Thrombosis is the medical term for when a blood clot — also known as a thrombus — develops inside a blood vessel and stays there. This impedes the flow of blood through the vessel.

An embolus is a piece of a thrombus that breaks off and travels through the blood vessel to other parts of the body. This is referred to as an “embolism.”

Globally, approximately 1 in 4 people are dying from conditions either associated with or caused by thrombosis, the International Society on Thrombosis and Haemostasis reports. Each year, the society says, thrombosis affects an estimated 900,000 people in the United States and causes around 100,000 deaths in the country.

There are two main types of thrombosis: arterial and venous.

Arterial thrombosis

This involves a thrombus developing in an artery. Arteries are blood vessels that carry oxygenated blood away from the heart to other areas of the body.

Cases of arterial thrombosis may be minor or severe, depending on the size of the thrombus and the artery in which it develops. A thrombus in a blood vessel that supplies blood to the brain may result in a stroke, and one in a blood vessel of the heart may cause a heart attack.

Venous thrombosis

This involves a thrombus developing in a vein. Veins are blood vessels that carry deoxygenated blood back to the heart.

A venous thromboembolism (VTE) is a broader term that describes blood clots in veins. There are two subtypes: deep vein thrombosis (DVT) and pulmonary embolism (PE).

DVT occurs when a thrombus, or clot, forms in a deep vein, usually within the arm, leg, or pelvis. PE occurs when part of a DVT breaks off and travels through the bloodstream to the lungs, causing a blockage in a blood vessel of the lungs.

Thrombosis interrupts blood flow through a blood vessel, reducing the amount of blood that reaches the parts of the body that the vessel supplies. The symptoms of thrombosis generally stem from a lack of oxygen in the affected body parts.


DVT typically develops in the deep veins of the legs, though it can develop in the pelvis or arms. Symptoms tend to occur in the affected limb or area and may include:

  • swelling
  • itchiness
  • throbbing, aching, or cramping pain
  • skin that is warm to the touch
  • skin discoloration or thickening

People with DVT may develop a condition called post-thrombotic syndrome in the following weeks or months. This condition may cause chronic symptoms in the affected limb or area that may include:

  • cramps
  • swelling
  • pain
  • skin discoloration


Symptoms of this type of embolism include:

Without treatment, a PE can cause severe breathing difficulties and even death. Anyone who may be having symptoms of a PE should receive emergency medical attention.

Optimal blood flow relies on a balance, or “homeostasis,” among these components of blood:

  • blood cells
  • plasma proteins
  • coagulation factors, which are proteins that help control bleeding
  • inflammatory cytokines, which are molecules that promote inflammation

A thrombus may develop as a result of an imbalance in these components. Or, it may result from issues affecting the blood vessels. A thrombus can also develop when blood flow slows significantly due to a long period of immobility.

Some other possible causes of blood clots include:

  • fractures
  • obesity
  • certain medications, such as those that contain estrogen
  • disease or injury to the deep veins in the arms, legs, or pelvis
  • autoimmune disorders that promote blood thickening, or coagulation
  • certain inherited disorders

Several factors can increase the risk of developing thrombosis. These include:

Some medical conditions that may increase the risk of blood clots include:

Before diagnosing thrombosis, a doctor will:

  • ask about the person’s symptoms, including when they began
  • review the person’s medical history, checking for past surgeries and current medications
  • ask whether there is a family history of thrombosis or related conditions

If the doctor suspects thrombosis, the following diagnostic tests can help them detect it:

  • blood tests to check for the protein fragment “D-dimer,” which is present when a blood clot dissolves
  • ultrasound scans to check for signs of DVT
  • CT scanning with intravenous dye to check for a PE
  • ventilation-perfusion scans, which use radioactive compounds to show parts of the lungs that are not receiving sufficient blood flow and oxygen

The most common and effective treatments for thrombosis are medications called anticoagulants. These help prevent the formation of new blood clots. Usually, a person receives injectable anticoagulants, such as heparin or low-molecular-weight heparin. These medications begin working within hours.

In emergency situations, a person with thrombosis may receive medications called tissue plasminogen activators. They promote the production of the enzyme plasmin, which is involved in dissolving clots.

Long-term treatment for thrombosis typically involves medications called direct oral anticoagulants. These block parts of the coagulation process. Examples of these drugs include warfarin (Coumadin, Jantoven) and rivaroxaban (Xarelto).

People who are unable to take anticoagulant medications may instead undergo surgery to have a filter placed inside the vena cava, which is a large vein in the abdomen. The filter prevents any blood clots from traveling to the heart and lungs.

The doctor may also recommend wearing compression stockings to encourage blood flow and help reduce the risk of long-term complications from thrombosis.

Treatment side effects

People taking anticoagulation medications have an increased risk of excessive bleeding. Signs of this issue include:

The following further increase the risk of bleeding complications in people taking anticoagulants:

  • being over 65 years of age
  • having liver or kidney failure
  • having cancer

People taking anticoagulant medications should receive emergency medical attention if they experience any of the following:

  • a major injury, such as from a car accident
  • a head injury
  • bleeding that will not stop

It is not always possible to prevent thrombosis. However, a person who knows that they have a risk of developing blood clots can take steps to reduce this risk.

Prevention in at-risk individuals

A doctor can perform a VTE risk assessment, and they should do this whenever a person is admitted to a hospital.

Anyone at risk of VTE typically receives anticoagulation medications to help prevent blood clots. They may also need to use devices that improve blood flow, such as compression stockings or intermittent pneumatic compression devices, which are cuffs that intermittently inflate around the legs.

General prevention tips

Other tips for preventing blood clots include:

  • moving or exercising the legs, especially:
    • after surgery
    • when staying in the hospital
    • when confined to bed
  • getting up and walking around every 2–3 hours while traveling
  • avoiding sitting for prolonged periods
  • performing gentle exercises when sitting for long periods, such as:
    • raising and lowering the toes with the heels on the floor
    • raising and lowering the heels with the toes on the floor
    • tightening and then relaxing the leg muscles
  • wearing loose-fitting, comfortable clothing, especially while sitting for prolonged periods
  • wearing compression clothing
  • maintaining a moderate body weight
  • getting enough exercise
  • treating chronic conditions
  • taking anticoagulants, when necessary

As with most medical conditions, early diagnosis and treatment of thrombosis greatly increases the chances of recovery and reduces the risk of long-term complications.

Long-term complications

In some cases, thrombosis resolves on its own, as the body breaks down and removes the thrombus. However, around 33–50% of people with DVT develop post-thrombotic syndrome. This stems from damage to the valves within the veins, which help direct blood flow.

Subsequent blood clots

Most people who develop thrombosis go on to have further or recurrent blood clots. However, the likelihood of this happening depends on the factors that caused the initial clot.

If a clot develops due to an injury or surgery, the chances of developing further clots is fairly low. If a clot develops due to an underlying health condition, the risk of another clot is fairly high.

In general, the risk of developing additional blood clots 6 months after completing treatment for a clot is 20% within the first 4 years and 30% after 10 years.

Thrombosis occurs when a blood clot forms inside a blood vessel and impedes blood flow it. There are two main types of thrombosis: arterial thrombosis, in which a blood clot blocks an artery, and venous thrombosis, in which a blood clot blocks a vein.

Thrombosis symptoms stem from a lack of blood and oxygen to the part of the body that the blocked vessel usually supplies.

Anyone who experiences symptoms of a blood clot should receive immediate medical attention. Early treatment reduces the risk of serious complications and improves the outlook.

Anyone at risk of blood clots should discuss preventive measures with a healthcare professional. These may include staying active, maintaining a moderate weight, and treating any underlying conditions that increase the risk of thrombosis.