Deep vein thrombosis (DVT): what it is, the symptoms, and what to do

What is a DVT?

DVT stands for deep vein thrombosis. It is a blood clot that forms inside one of the deep veins of the body, most often in the calf or thigh. Deep veins are the large veins that run through the muscles of your legs, not the ones you can see just beneath the skin.

Blood clots form when blood becomes thick and sticky in a vein. Think of it like traffic coming to a standstill: when blood stops moving properly, it can begin to clump together and form a solid mass. This clot can block part or all of the vein, causing the leg to swell and ache.

DVT affects roughly 1 to 2 people in every 1,000 each year in the UK. It is more common than many people realise, and it is also very treatable when caught promptly.

What causes a DVT?

DVT usually happens when one or more of three things occur: blood slows down or stays still in a vein, the lining of the vein is damaged, or the blood itself clots more easily than normal.

Common situations that raise your risk include:

• Long periods of immobility, such as a long-haul flight or a long car journey
• A stay in hospital, particularly following surgery
• Having a plaster cast on a leg
• Pregnancy and the weeks after giving birth
• Active cancer or cancer treatment
• Being significantly overweight
• Dehydration
• An inherited tendency for the blood to clot more easily than normal (thrombophilia)
• Previous DVT
• Taking the combined contraceptive pill or hormone replacement therapy (HRT)

In some people, a DVT occurs without any obvious trigger. This is called an unprovoked DVT, and it may prompt your doctor to look for an underlying cause.

What does a DVT feel like?

Symptoms vary from person to person, and some people have no symptoms at all. When symptoms do occur, they usually affect one leg rather than both. The most common signs are:

• Aching or throbbing pain in the calf or thigh, which may worsen when you stand or walk
• Swelling in one leg, ankle or foot
• Skin that feels warm to touch over the affected area
• Redness or discolouration of the skin, typically on the back of the leg below the knee

These symptoms can also be caused by other conditions, such as a muscle strain or a skin infection, which is why proper medical assessment is important. Do not try to diagnose yourself.

How is a DVT diagnosed?

If your doctor suspects a DVT, they will usually carry out a combination of assessments. There is no single test that diagnoses DVT on its own.

The first step is often a clinical scoring system called the Wells score. This is a checklist of symptoms and risk factors that gives a probability of whether a DVT is likely. Based on this score, your doctor will decide which tests are needed next.

A D-dimer blood test measures a substance that is released when a blood clot breaks down. A negative D-dimer result in someone with a low Wells score can rule out DVT without the need for a scan. However, D-dimer levels can be raised by many things other than DVT, so a positive result alone does not confirm a clot.

The main diagnostic test is a Doppler ultrasound scan of the leg. This uses sound waves to produce a picture of the veins and can show whether a clot is present. According to NICE guideline NG158, if a DVT is suspected, an ultrasound should be performed with the result available within 24 hours.

How is a DVT treated?

The main treatment for DVT is anticoagulation, which means medication that prevents the clot from growing and reduces the risk of it breaking off. These are commonly called blood thinners, though they do not actually thin the blood. They work by slowing down the clotting process.

NICE guideline NG158 recommends apixaban or rivaroxaban as the first-choice treatment for most people with DVT. Both are taken as tablets and do not require regular blood tests, which makes them much more convenient than older treatments such as warfarin.

For most people, treatment lasts at least 3 months. If your DVT was caused by a clear temporary trigger, such as surgery or a long flight, you may only need 3 months of treatment. If no obvious cause was found, your doctor may recommend continuing for 6 months or longer, weighing up your risk of another clot against your risk of bleeding. This decision is made together with you.

Most people with DVT are treated as outpatients and do not need to be admitted to hospital. You can usually start your medication the same day.

Are there other treatments for DVT?

For most people, blood-thinning tablets are all that is needed. However, in some cases a more active approach may be considered.

Catheter-directed thrombolysis (CDT) is a procedure in which a thin tube is guided into the clot and a drug is delivered directly to break it down. This is usually considered for people who have a large clot in the veins of the pelvis or upper thigh (called an ilio-femoral DVT), where there is a higher risk of long-term leg problems, and where symptoms started less than 14 days ago.

In rare cases, a mechanical device may be used alongside CDT to help remove the clot more quickly. This is called pharmacomechanical thrombolysis.

If you are a candidate for any of these treatments, a vascular surgeon will discuss the options, risks and benefits with you in detail.

What happens if a DVT is left untreated?

Without treatment, there are two main risks. The first is that the clot grows larger and causes more severe leg symptoms, including persistent swelling and pain.

The second, more serious risk is that part of the clot breaks away and travels to the lungs, causing a pulmonary embolism. A large PE can be life-threatening. This is the main reason why prompt treatment matters.

Even with treatment, some people develop post-thrombotic syndrome in the months or years after a DVT. This is a condition where the vein is left damaged by the clot, causing ongoing leg aching, heaviness and sometimes swelling. Keeping active, maintaining a healthy weight, and attending all your follow-up appointments can help reduce this risk.

What should you do next?

If you have leg pain and swelling, particularly in just one leg, do not wait to see if it gets better on its own. Contact your GP today, or go to your nearest urgent treatment centre. If you also have difficulty breathing or chest pain, call 999 immediately.

If you are concerned about vascular symptoms, or if you would like a private consultation with a vascular surgeon for another condition, appointments are available at short notice in Birmingham and Solihull. You do not need a GP referral.

DVT is a condition that responds well to the right treatment. The large majority of people treated promptly make a full recovery.

Iliofemoral DVT: a more serious form that needs specialist care

Not all DVTs are the same. A clot that extends into the large veins of the pelvis, including the common femoral vein or the iliac veins, is called an iliofemoral DVT. This is a more extensive clot than one confined to the calf or thigh, and it tends to cause more severe swelling and pain. It also carries a higher risk of developing long-term leg problems.

Post-thrombotic syndrome is the medical term for chronic leg heaviness, aching and swelling that can persist for months or years after a DVT. It occurs when the clot damages the valves inside the vein, causing blood to pool in the leg. Studies suggest that between 20% and 50% of people with a proximal DVT develop some degree of post-thrombotic syndrome. In its more severe form, it can significantly affect daily life.

For this reason, patients with an iliofemoral DVT are sometimes considered for more active treatment beyond blood thinners alone, with the aim of removing the clot and reducing the risk of long-term complications.

Who might be considered for clot removal?

Not every patient with an iliofemoral DVT needs an intervention. Blood-thinning medication alone remains the standard treatment for most people. However, specialist assessment is recommended when the following features are present:

• Clot involving the iliac veins or the common femoral vein
• Symptoms that started within the past 14 days (acute clot)
• Severe leg swelling or pain that is significantly affecting daily activity
• Good general health and life expectancy
• Low risk of bleeding complications

The decision is made on an individual basis, weighing the potential benefit of clot removal against the risks of the procedure. A vascular surgeon will discuss the options with you in detail.

Catheter-directed thrombolysis (CDT)

Catheter-directed thrombolysis (CDT) is a procedure in which a thin, flexible tube (catheter) is guided through the blood vessels and positioned directly inside the clot. A clot-dissolving drug called a thrombolytic agent is then delivered through the catheter over several hours or days, breaking down the clot from the inside out.

CDT is more targeted than giving thrombolytic drugs directly into a vein, which reduces the overall dose needed and lowers the risk of bleeding elsewhere in the body. However, it does carry a small but real risk of serious bleeding, including rare cases of bleeding in the brain.

The evidence for CDT in iliofemoral DVT is mixed. The largest randomised trial, known as ATTRACT, found that adding pharmacomechanical CDT to anticoagulation did not significantly reduce the overall rate of post-thrombotic syndrome at two years compared to anticoagulation alone. However, a closer analysis of patients in that trial with clots specifically in the iliac or common femoral veins did suggest some benefit in that subgroup. An earlier Norwegian trial, CaVenT, did show a reduction in post-thrombotic syndrome at both two and five years with CDT, particularly in patients with iliofemoral involvement.

CDT is currently used selectively at specialist centres for patients with severe acute iliofemoral DVT who meet the right criteria, accepting that the overall evidence base remains incomplete.

Mechanical thrombectomy: the newer approach

Mechanical thrombectomy is a newer technique in which a specialised device is guided into the vein and used to physically remove the clot, without the need for thrombolytic drugs. Because no clot-dissolving medication is used, the risk of bleeding complications is lower than with CDT.

Several devices are available. One of the most studied is the ClotTriever system, which uses a mesh retrieval mechanism to extract clot in a single session. The largest published dataset, the CLOUT registry, enrolled 500 patients across 43 centres in the United States. In-hospital results showed that more than 9 in 10 patients achieved near-complete clot removal, with a very low rate of device-related serious adverse events at 0.2%. Patients reported significant and immediate relief from pain and swelling. Two-year outcomes were presented in 2024, adding to the growing evidence base.

Mechanical thrombectomy does not yet have an evidence base from a large randomised controlled trial comparing it head-to-head against anticoagulation alone for long-term outcomes such as post-thrombotic syndrome. It is therefore currently used at specialist centres for selected patients, with ongoing research to better define who benefits most.

Venous stenting

In some patients, an iliofemoral DVT is triggered or worsened by a compression of the iliac vein from a surrounding structure. The most common example is May-Thurner syndrome (also called Cockett syndrome), where the right common iliac artery lies across the left common iliac vein, pressing on it and narrowing the blood flow. This compression can predispose to DVT and, if left uncorrected, increases the risk of the vein reblocking after treatment.

If imaging after clot removal reveals a significant underlying narrowing of the vein, a stent may be placed to prop the vein open and restore normal flow. Dedicated venous stents designed specifically for this purpose have been available since 2019 and are increasingly used at specialist centres.

Venous stenting is also used in patients who develop chronic obstruction of the iliac veins months or years after a DVT, resulting in persistent post-thrombotic syndrome. In this setting, expert consensus guidance suggests that stenting can be considered when post-thrombotic symptoms are moderate to severe, when they significantly limit daily life, and when at least six months of anticoagulation has not resolved the obstruction.

Stent patency rates in the iliac veins are generally good. In a study of patients treated for May-Thurner-related DVT, 77% maintained patency at two years. Long-term anticoagulation is usually continued after stenting to protect the stent from re-thrombosis.