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What is deep vein thrombosis (DVT)?

What is deep vein thrombosis (DVT)?

Deep vein thrombosis (DVT) refers to a blood clot (thrombus) in a deep vein in your body.

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23 October 2025

What is deep vein thrombosis (DVT)?

Deep vein thrombosis (DVT) refers to a blood clot (thrombus) in a deep vein in your body and is a potentially life-threatening condition. 

It is a type of thromboembolism. Thromboembolism refers to the blockage of a blood vessel by a blood clot and affects 1–2 in every 1,000 people. 

Here, we will explore how deep vein thrombosis occurs, and its symptoms, risk factors, consequences and treatment. 

How does deep vein thrombosis occur?

Blood clots are thick, gel-like structures made from protein, platelets and other components in your blood. They stop bleeding and help wounds heal; they are then broken down or dissolved. 

However, blood clots can also form when they are not needed and can fail to dissolve as quickly as they should. This is what happens in DVT. 

Slow blood flow through your veins makes it more likely that components in your blood will thicken and stick together. This allows one or several blood clots to form and stick to the inner wall of a deep vein, resulting in DVT.

DVT is also more likely if a vein is damaged, or you have a blood disorder that means your blood does not clot as it should. A thrombosis or blood clotting test can check whether your blood clots properly.  

DVT usually occurs in the deep veins of the leg. This may include the femoral vein in the thigh and the popliteal vein behind the knee. DVT also commonly occurs in the iliac veins of the pelvis. 

Symptoms of deep vein thrombosis

As DVT can be life-threatening, it is important to be aware of its symptoms so you can seek urgent help when needed. When deep vein thrombosis affects the leg, it can cause: 

  • Severe cramping pain in the leg
  •  Swelling of the leg
  •  Swollen, sore and hard veins around the affected area
  •  Throbbing pain when standing or walking 
  •  Warm skin that is red, blue-tinged or darkened around the affected area

DVT less commonly affects the upper body or arms. In these cases, it often does not cause any symptoms. If symptoms do occur, they can include: 

  • Neck and/or shoulder pain
  •  Pain that travels along the arm
  •  Swelling of the arm and/or hand
  •  Weakness of the hand

If you notice any of these symptoms, call 111. If you experience any chest pain or breathlessness, call 999 or go to A&E as you may have a pulmonary embolism, which is life-threatening.

A pulmonary embolism occurs when part of the blood clot trapped in a deep vein comes loose and travels to your lungs, where it blocks one of your pulmonary arteries (ie one of the blood vessels that supplies oxygen-rich blood to your lungs). This can lead to a heart attack or stroke. For every 100 people who have a pulmonary embolism, it is fatal in around 1–3 people. 

Risk factors for DVT

Certain aspects of your health and your circumstances can increase your risk of DVT. You are more likely to develop deep vein thrombosis if you are aged over 60 and/or are overweight, and if you have certain health conditions, including:

Your risk is also higher if you have had DVT before, have a family history of DVT, or if you are pregnant or gave birth in the preceding 6 weeks. 

Lifestyle factors that increase your risk include smoking, taking hormone replacement therapy (HRT) or the contraceptive pill, and being dehydrated.

Your present circumstances also affect your risk. The chances of DVT rise if you are sedentary. For example, your risk rises if you are bed-bound, in hospital or recovering at home from an operation, or if you are on a journey longer than 3 hours where you are seated (eg on a plane or train, or in a car).

While all of these factors contribute to your chances of developing DVT, sometimes deep vein thrombosis can occur without any obvious cause. 

Getting a diagnosis

If you suspect you may have deep vein thrombosis, seek urgent medical help. Your doctor will ask about your symptoms and physically examine the affected area. If they think you have DVT, you will be referred to have an ultrasound scan within 24 hours and may also have an X-ray with a contrast agent. 

The ultrasound scan will check the blood flow through the vein suspected to have DVT. 

If clear enough images cannot be obtained via an ultrasound scan, you will have an X-ray with a contrast agent. The contrast agent helps create clearer images. If DVT is suspected in your leg, the contrast agent will be injected into a vein in your foot or ankle. 

Deep vein thrombosis treatments

Treatment for DVT may be started before your ultrasound scan or X-ray to confirm its presence based on your symptoms. This involves having a heparin injection – heparin is an anticoagulant medication that prevents blood clots from forming and prevents existing blood clots from growing larger. However, heparin does not dissolve existing blood clots.

Once deep vein thrombosis is confirmed, you will be asked to take anticoagulant medication in the form of tablets, such as warfarin or rivaroxaban, for up to 3 months. Taking this medication will give your body time to safely break down and remove your DVT.

In severe cases of DVT, where the blood clot is very large or causing significant damage to the surrounding tissue, your doctor may recommend surgery called a surgical thrombectomy. This involves cutting into the affected vein, removing the clot and repairing the vein. 

If you develop deep vein thrombosis during pregnancy, you will receive daily heparin injections for the remainder of your pregnancy and the first 6 weeks after birth. Heparin injections do not affect the baby and do not prevent you from breastfeeding.

Recovering from deep vein thrombosis

As you recover from deep vein thrombosis, you will need to stay active by walking regularly. 

If your leg is affected by DVT, keep it elevated whenever you are sitting to improve its blood flow. Also, for at least 2 weeks, you should avoid any journeys longer than 3 hours where you will be seated. 

How to prevent deep vein thrombosis

You can reduce your risk of DVT by reducing the risk factors that you can control. For example, by maintaining a healthy weight, exercising regularly, quitting smoking and not drinking excessive amounts of alcohol. 

You should also stay hydrated throughout the day and try to avoid sitting for more than an hour at a time. When you are seated, do not cross your legs as this impairs blood flow through the crossed leg. 

If you are going on a long journey where you will be seated, you can wear compression stockings and practice toe and ankle exercises while seated to improve blood flow in your legs. These exercises involve moving your toes up and down and rotating your ankles in a circular motion. Make sure you walk around whenever possible and ideally at least every hour.