Deep vein thrombosis (DVT)

What is a deep vein thrombosis, and how common is it?

A deep vein thrombosis (DVT, or venous thrombosis) is a blood clot in a deep vein, usually in the leg¹. It can also form in the arms or abdomen¹.

DVT affects about 1 in 1000 people². It becomes more common with age³.

A clot below the knee was long thought to be harmless, but it can grow, come back, travel to the lungs or lead to long-term leg problems⁴. If a DVT isn't treated, the clot can travel to the lungs². This is called a pulmonary embolism, and it can be life-threatening².

What causes a DVT, and who is at risk?

A DVT usually starts when blood flow in the deep veins slows, a blood vessel is injured, or something makes your blood clot more easily¹.

Common risk situations include:

• remaining seated for long periods, for example on a journey of 4 hours or more by air, rail or road⁵, or being confined to bed in a hospital or nursing home⁶;
• surgery, particularly bone and joint (orthopaedic) surgery, with the chance of a clot highest in the first 3 months and lowering with time⁶⁷;
• major injury, infection and inflammation⁸;
• cancer and its treatments, pregnancy and older age⁶⁷;
• being overweight, smoking, a previous clot, or an inherited clotting condition¹⁶.

A DVT can also happen with no obvious cause, so having no risk factors doesn't rule it out¹. For people having surgery or with several risk factors, preventing clots is a topic that can help.

What does a DVT feel like, and how is it diagnosed?

A DVT often causes throbbing pain and swelling in one leg (rarely both), usually in the calf or thigh⁹. The leg may feel tender and warm, and the skin can look red, blue or darker⁹¹⁰. The colour change may be harder to see on brown or black skin⁹. Some people have no symptoms¹.

Symptoms alone can't confirm a DVT¹¹. A doctor checks how likely it is, then arranges a D-dimer blood test and an ultrasound scan³. A blood test can rule a DVT out only when a probability score shows the risk is low, not on its own when the risk is high, and an ultrasound scan confirms the diagnosis³¹¹.

A ruptured aneurysm (a bulging artery) behind the knee can look like a DVT and is often missed at first, delaying treatment¹². So a sore, swollen leg needs proper assessment, not guessing.

How is a DVT treated first?

A DVT needs medical care from the start, not self-care.

The main treatment is blood thinners (anticoagulants)¹³. They're usually tablets, taken for at least 3 months to lower the risk of another clot⁹. Take the full course your doctor sets⁹.

Direct oral anticoagulants (tablet blood thinners) probably work about as well as conventional blood-thinning treatment at preventing further clots, and they may cause less major bleeding². Bleeding is still a limit of treatment, particularly for older people and people with other health conditions¹⁴.

Walking or swimming, done gently, is the kind of exercise that is advised¹⁰. While you're on blood thinners, avoid contact sports and other activities that may cause injury or bruising¹⁰.

What helps if that isn't enough?

Blood thinners are the main treatment for DVT¹³¹⁴. When a clot is life- or limb-threatening, clot-dissolving treatment may be used as well as blood thinners¹³. Clot-dissolving treatment or a suitable alternative is also used for a massive clot in the lungs that is affecting circulation¹¹. The hospital team makes this decision, not you.

How long does recovery take, and can a DVT come back?

Many people do well with treatment, but a DVT can come back: up to 1 in 4 people who've had a DVT or pulmonary embolism have another clot⁷. A clot below the knee isn't harmless⁴.

Some people are left with post-thrombotic syndrome (long-term leg pain, swelling and heaviness after a DVT), which is common and can lead to leg ulcers if severe¹⁵.

To lower the chance of another clot:

• Finish your blood-thinner course and go to follow-up⁹.
• Stay gently active, and keep moving on journeys of 4 hours or more.
• Preventive treatment is relevant before surgery or when several risk factors are present¹⁴.

What can make it serious?

• Your calf or leg on one side becomes swollen and painful, especially if it is also warm or red.
• You suddenly become short of breath, have chest pain or cough up blood.
• You have bleeding that won't stop when you press firmly on it.