About Deep Vein Thrombosis
Deep vein thrombosis is a blood clot within the deep veins of the leg, which may involve the calf, the thigh, or both. It can occur spontaneously or follow a provoking event such as surgery, trauma, immobility or illness. Some clots resolve on their own, while others extend into the veins above the knee, where the risk of complications is higher. The most serious immediate complication is pulmonary embolism, where part of the clot travels to the lungs. In the longer term, damage to the vein valves can lead to post-thrombotic syndrome, with chronic leg swelling, aching and in some cases ulceration. DVT is treatable, and prompt diagnosis matters.
Deep Vein Thrombosis DVT
What is deep vein thrombosis?
Deep vein thrombosis, or DVT, is a blood clot that forms in one of the deep veins, usually in the calf or thigh. The deep veins carry the great majority of the blood returning from the leg, so a clot in them behaves quite differently from a clot in a surface vein.
Clots most often begin in the calf veins. Some resolve as the body breaks them down naturally; others extend upwards into the popliteal and femoral veins, and occasionally into the pelvic and abdominal veins. A clot in the veins above the knee is described as a proximal DVT and carries a higher risk of complications than one confined to the calf.
DVT and pulmonary embolism together are known as venous thromboembolism, or VTE. It is a common condition and it is treatable, but it needs to be diagnosed properly rather than assumed either way.
Symptoms of DVT
A DVT may cause:
- swelling of the calf or the whole leg, usually on one side only;
- calf or thigh pain or tenderness, often worse on standing or walking;
- warmth and redness or discolouration of the skin;
- surface veins that become more prominent than usual.
Importantly, a substantial proportion of DVTs cause few or no symptoms, and many other conditions produce a swollen, painful calf. Neither the presence nor the absence of symptoms is reliable on its own, which is why DVT cannot be diagnosed or excluded on clinical grounds alone12.
Seek medical attention promptly if you develop unexplained swelling or pain in one leg. Seek emergency care if you develop breathlessness, chest pain that is worse on breathing in, coughing up blood, or you feel faint, as these may indicate a pulmonary embolism.
Why DVT happens
Clotting is favoured by three factors, described over a century ago and still valid: sluggish blood flow, injury to the vein wall, and an increased tendency of the blood to clot.
Recognised risk factors include:
- recent surgery, particularly orthopaedic surgery to the hip or knee;
- immobility, including hospital admission, plaster casts and long-haul travel;
- cancer and chemotherapy;
- pregnancy and the weeks after delivery;
- the combined oral contraceptive pill and hormone replacement therapy;
- increasing age, obesity, smoking and dehydration;
- trauma or injury to the leg;
- a previous DVT or a family history of clots;
- inherited or acquired clotting disorders.
Where a clear precipitant is present, the DVT is described as provoked. Where none is found it is described as unprovoked, and this influences how long anticoagulation is continued12.
How DVT is diagnosed
Assessment combines clinical scoring, a D-dimer blood test and ultrasound. A validated clinical score, such as the Wells score, estimates how likely a DVT is. Where the probability is low, a negative D-dimer can reliably exclude it without further imaging. Where the probability is higher, or the D-dimer is raised, a proximal leg vein ultrasound scan is performed12.
The scan is painless, uses no radiation and no needles, and shows directly whether a vein is compressible and whether blood is flowing normally through it. Where a first scan is negative but suspicion remains, the scan is repeated after several days.
For people with an unprovoked DVT who are not known to have cancer, current guidance is to review the medical history and baseline blood results and to perform a physical examination. Extensive routine screening for hidden cancer is no longer recommended in the absence of relevant symptoms or signs, because trials showed it did not improve outcomes while causing cost, radiation exposure and anxiety2. Further investigation is appropriate where symptoms or signs suggest it.
Treatment of DVT
Prompt diagnosis and treatment reduce the risk of pulmonary embolism, of the clot extending, and of long-term complications in the leg.
Anticoagulation is the mainstay of treatment. It does not dissolve the existing clot; it prevents it enlarging and prevents new clots, while the body breaks down what is already there. Direct oral anticoagulants such as apixaban and rivaroxaban are now generally recommended as first line treatment for confirmed DVT, with warfarin and low molecular weight heparin used where they are more appropriate, for example in pregnancy or in some patients with cancer12.
How long treatment continues depends on why the clot occurred. A provoked DVT is usually treated for around three months. An unprovoked DVT, a recurrent DVT, or a DVT in someone with ongoing risk factors may warrant longer or indefinite anticoagulation, balancing the risk of another clot against the risk of bleeding12.
Early mobilisation is encouraged. Bed rest is not necessary or beneficial once anticoagulation has been started.
When intervention is considered
The great majority of patients are managed with anticoagulation alone. A small number are considered for something more.
Catheter-directed thrombolysis delivers clot-dissolving drugs directly into the clot through a fine catheter. It is reserved for selected patients, typically those with extensive iliofemoral DVT, severe symptoms of recent onset, a long life expectancy and a low bleeding risk2. The aim is to relieve severe acute symptoms and, in some patients, to reduce the risk of severe post-thrombotic syndrome later. It carries a meaningful bleeding risk, so the decision is individual.
Inferior vena cava filters are devices placed in the main abdominal vein to catch clot travelling towards the lungs. Evidence of benefit is limited, and current guidance restricts their use to patients in whom anticoagulation is contraindicated or has failed during treatment2. Where one is placed, it should be removed once anticoagulation can safely be started.
Surgical or endovascular treatment of chronic obstruction is occasionally appropriate much later, in patients with severe post-thrombotic symptoms and a blocked iliac vein. This is a specialised area and requires assessment at a centre offering deep venous intervention.
These decisions rest with the physician or vascular specialist managing your DVT. Our role is usually in assessing and treating the superficial venous disease that may follow.
Follow-up after a DVT
A DVT is not finished when the pain settles. Follow-up matters for three reasons.
Reviewing how long to anticoagulate. Around the three month mark, the doctor managing your treatment will reassess whether to stop or continue, weighing your risk of another clot against your bleeding risk12. This is a decision to revisit rather than one made once at diagnosis, and it should be an active conversation.
Monitoring for post-thrombotic syndrome. Symptoms usually develop within the first two years. Persistent swelling, aching, heaviness or skin changes in the affected leg should be reported rather than accepted as permanent, because they can be assessed and managed.
Assessing the veins. Where leg symptoms persist, a duplex ultrasound scan establishes whether the deep veins have recanalised, whether deep venous reflux has developed, and whether any superficial reflux is contributing. That assessment determines what can usefully be treated, and is what we do at our vein centres.
Tell any doctor treating you that you have had a DVT, particularly before surgery, pregnancy or a long period of immobility, as preventive treatment may be indicated. Seek urgent attention for new leg swelling, or for breathlessness or chest pain.
Compression stockings after DVT
The role of compression after DVT has changed as better evidence has emerged, and it is worth being precise about it.
Compression stockings were once given routinely after a DVT to prevent post-thrombotic syndrome. A large placebo-controlled randomised trial subsequently found no benefit for that purpose4, and NICE guidance now advises against offering compression stockings routinely to prevent post-thrombotic syndrome or recurrent clots after a proximal DVT2.
That is a narrower statement than it may first appear. Compression remains useful for relieving symptoms — the swelling, heaviness and aching that follow a DVT — and for managing established post-thrombotic syndrome23. Many patients find their leg is considerably more comfortable in a stocking, and that is a legitimate reason to wear one.
So compression is offered for symptoms rather than prescribed to everyone as prevention. Whether it will help you is a discussion to have with the doctor managing your anticoagulation. See our page on compression stockings for how they work and which class is used.
Long-term complications
Pulmonary embolism occurs when part of the clot breaks away and travels to the lungs. This is the most serious immediate complication and the reason DVT is treated urgently.
Post-thrombotic syndrome describes the chronic leg problems that can follow a DVT: swelling, aching, heaviness, skin discolouration and, in severe cases, venous ulceration. It develops in a significant proportion of patients after a proximal DVT, though reported rates vary widely between studies depending on definition and length of follow-up3. It arises because the clot damages the vein valves, producing deep venous reflux.
Post-thrombotic syndrome matters for vein treatment generally. Patients with significant deep venous reflux following a DVT may not be candidates for treatment of their superficial veins, and this is one of the things a duplex scan establishes before any vein procedure is planned. Management is usually compression, leg elevation, exercise and skin care; a minority of patients benefit from intervention on the deep veins.
Reducing your risk
Not every DVT is preventable, but risk can be reduced:
- keep moving during long flights or car journeys, and stay well hydrated;
- walk regularly and avoid prolonged immobility where you can;
- follow the thromboprophylaxis advice given around surgery or hospital admission, which is the single most effective preventive measure;
- maintain a healthy weight and stop smoking;
- discuss your risk with your doctor before starting the combined pill or HRT if you have had a clot or have a family history.
If you have had a previous DVT, tell any doctor treating you, particularly before surgery or a long period of immobility. Recurrence is a real risk and prophylaxis may be indicated.
References
- Stevens SM, Woller SC, Kreuziger LB, et al. Antithrombotic therapy for VTE disease: second update of the CHEST guideline and expert panel report. Chest. 2021;160(6):e545–608. PubMed 34352278
- National Institute for Health and Care Excellence. Venous thromboembolic diseases: diagnosis, management and thrombophilia testing. NICE guideline NG158. 2020 (updated 2023). NICE NG158
- De Maeseneer MG, Kakkos SK, Aherne T, et al. European Society for Vascular Surgery (ESVS) 2022 clinical practice guidelines on the management of chronic venous disease of the lower limbs. Eur J Vasc Endovasc Surg. 2022;63(2):184–267. ESVS guideline
- Kahn SR, Shapiro S, Wells PS, et al; SOX trial investigators. Compression stockings to prevent post-thrombotic syndrome: a randomised placebo-controlled trial. Lancet. 2014;383(9920):880–8. PubMed 24315521
Written and medically reviewed by Dr Johan Blignaut, MBChB (UOFS), FCS(SA), Specialist Surgeon.
Last medically reviewed: August 2026. This page is reviewed periodically and updated when clinical practice changes. It is general information, not medical advice – see our terms and medical disclaimer.