A blood clot in the deep veins of the leg is called a “deep vein thrombosis” or DVT; if a piece of it breaks off and reaches the lung, it causes a “pulmonary embolism”, which can be dangerous [1]. The main sign of a DVT is swelling and pain in one leg, and the main sign of a pulmonary embolism is sudden breathlessness or chest pain; with these signs, call your local emergency number straight away [1]. Treatment is a blood thinner (anticoagulant) for at least 3 months and, if the risk of complications is low, it can usually be given at home [2, 11].
Key points
- Blood clots most often form in the deep veins of the calf or thigh (DVT); if a piece breaks off and reaches the lung, it causes a “pulmonary embolism” [1]. Each year about 1 to 2 in every 1,000 people get one [2].
- The risk is higher after surgery, a hospital stay or a period of immobility, in pregnancy and for up to 3 months after giving birth, with oestrogen-containing contraceptive pills, with cancer and its treatment, and in people who have had a clot before [3].
- Take swelling, pain and warmth in one leg seriously and see a doctor soon. Sudden breathlessness or chest pain is an emergency: call your local emergency number [1].
- The diagnosis is made with an examination, a D-dimer blood test and a Doppler ultrasound scan of the leg or a CT scan of the lung arteries (CT pulmonary angiogram) [9, 10].
- The main treatment is a blood thinner (anticoagulant) for at least 3 months; whether it continues after that depends on what caused the clot [2, 11].
- On journeys longer than 4 hours, get up and walk every 1 to 2 hours and move your legs while you are sitting [1].
What are a DVT and a pulmonary embolism?
Blood clots so that bleeding stops after an injury. But sometimes a clot forms where it should not, inside a blood vessel. When a clot forms in one of the deep veins, it is called a “deep vein thrombosis” or DVT. Deep veins are large veins that carry blood back to the heart and cannot be seen through the skin. The clot is usually in the calf, thigh or pelvis, and sometimes in the arm [1].
The main danger is that a piece of the clot breaks off, travels with the blood to the lung and blocks a blood vessel there. This is called a “pulmonary embolism”. A pulmonary embolism can be life-threatening and needs urgent treatment [1]. Doctors call the two together “venous thromboembolism” (VTE) [1].
This clot is different from the clots that cause a heart attack or stroke. Those clots form in arteries, and a DVT does not cause a heart attack or stroke [1]. A blood clot in the leg is not cancer either. An anticoagulant stops the clot from growing and new clots from forming, and in the meantime the body slowly absorbs the clot [9].
Some people have swelling, pain or skin discolouration in the leg for a long time after a DVT. This is called “post-thrombotic syndrome”. It affects 2 to 5 in every 10 people, and in a few it is severe [2]. After a pulmonary embolism, a few patients are left with high blood pressure in the lung arteries [2].
Who is most at risk?
Almost anyone can get a blood clot, but some situations raise the risk. When several of them come together, the risk is higher still [3].
Surgery, broken bones and hospital stays
Long journeys and sitting for long periods
Pregnancy, the pill and hormones
Pregnancy raises the risk of a clot about fivefold, and the higher risk lasts until 3 months after giving birth [5]. Oestrogen-containing contraceptive pills, patches and rings and menopausal hormone therapy also raise the risk [3, 4]; even so, the risk of a clot with the pill is very low for any healthy woman [6].
Cancer and its treatment
About 1 in every 5 clots is linked to cancer and its treatment, and the risk is highest in the first few months after diagnosis, which is when treatment usually takes place [8]. See the section “Blood clots and cancer”.
A previous clot and other factors
A previous clot, an inherited clotting disorder, a family history of clots, older age, being overweight, heart or lung disease, inflammatory bowel disease and a catheter (a thin tube placed in a large vein) [3].
Symptoms
About 1 in 2 people with a DVT has no symptoms at all. When there are symptoms, they are usually in one leg (or one arm) [1]:
- swelling
- pain or tenderness
- skin that feels warm
- redness or a change in skin colour
If you have these symptoms, see a doctor as soon as possible [1].
A pulmonary embolism can happen without any symptoms in the leg. Its symptoms are [1]:
- breathlessness, especially if it starts suddenly
- a fast or irregular heartbeat
- chest pain or discomfort that usually gets worse when you breathe in deeply or cough
- a cough, or coughing up blood
- very low blood pressure, light-headedness or fainting
With these symptoms, call your local emergency number straight away [1].
Not every swollen leg is a clot. A muscle injury, a skin infection (cellulitis) and inflammation of the surface veins just under the skin can cause similar symptoms; this is why the diagnosis is made only with tests and scans [9]. If your arm or leg has swollen after cancer surgery or radiotherapy, also read Lymphoedema: swelling of the arm or leg after cancer treatment; but always show a new, one-sided swelling to a doctor.
Which tests?
The doctor first asks about your symptoms and risk factors and examines you to judge how likely a clot is; the tests are then chosen according to that likelihood [10].
- D-dimer blood test: measures a substance released into the blood when a clot breaks down. If it is negative, a clot is unlikely [9]. A raised D-dimer does not prove a clot on its own, because it has other causes too; a scan is then needed [11].
- Doppler ultrasound of the leg: uses sound waves to show the blood flow in the veins and is the main test for diagnosing a DVT [9].
- CT pulmonary angiogram: a CT scan with an injection of contrast dye into a vein that shows the blood vessels of the lungs; it is the main test for diagnosing a pulmonary embolism [9].
- Lung scan (ventilation–perfusion or V/Q scan): used when a CT pulmonary angiogram is not available or not suitable for the patient [9].
Before a blood thinner is started, basic blood tests are also done: a full blood count (CBC), kidney and liver tests and clotting tests [11].
For physicians
| Pretest probability | Pulmonary embolism (PE) | Lower-limb DVT |
|---|---|---|
| Low (PE 5% or less, DVT 10% or less) | Start with D-dimer; if needed, then V/Q scan or CTPA (strong recommendation) [10] | Start with D-dimer; if needed, then proximal or whole-leg ultrasound (strong recommendation) [10] |
| Intermediate (PE about 20%, DVT about 25%) | Start with D-dimer; if needed, then V/Q scan or CTPA (conditional recommendation) [10] | Whole-leg ultrasound, or proximal ultrasound repeated serially if negative and no alternative diagnosis is found; in lower-prevalence populations, starting with D-dimer is also acceptable (conditional recommendation) [10] |
| High (50% or more) | Start with CTPA (conditional recommendation) [10] | Start with proximal or whole-leg ultrasound, followed by serial ultrasound if negative and no alternative diagnosis is found (conditional recommendation) [10] |
NICE: two-level Wells score; if the ultrasound or CTPA result cannot be obtained within 4 hours, start interim therapeutic anticoagulation; with a negative proximal ultrasound and a positive D-dimer, repeat the ultrasound 6 to 8 days later; consider an age-adjusted D-dimer threshold in people over 50 [11].
Treatment
The main treatment is a blood thinner (anticoagulant). These medicines do not really thin the blood; they stop the clot from growing and new clots from forming, so that the body can slowly clear the clot itself [9]. Today most patients take one of the newer tablets, such as apixaban or rivaroxaban; enoxaparin injections and warfarin tablets are also common options [2, 11]. How to take each medicine and how to take it safely is explained in Blood thinners.
Home or hospital? If the DVT is uncomplicated, or the pulmonary embolism is low-risk, treatment at home is usually possible [2, 11]. A severe pulmonary embolism that has lowered the blood pressure is treated in hospital, sometimes with a clot-dissolving medicine [2, 9]. If someone cannot take an anticoagulant, a small filter is sometimes placed in the large vein in the abdomen to stop clots reaching the lungs [9, 11].
Walking: once the medicine has been started, you usually do not need to stay in bed. In studies, walking early while taking an anticoagulant did not raise the risk of a clot reaching the lung and eased leg pain [13].
Compression stockings: they may ease swelling and pain in the leg, but for most patients they do not prevent long-term complications; this is why guidelines do not recommend them for everyone. If your leg is swollen or painful, ask your doctor about them [2, 11].
How long does treatment last? At least 3 months [11, 12]. After that, the decision depends on what caused the clot [2, 11]:
- If the clot had a clear, temporary cause, such as surgery, a broken bone or pregnancy, the medicine is usually stopped after 3 to 6 months [2, 11].
- If the clot had no clear cause, or its cause is still present (for example active cancer), long-term treatment is usually suggested and reviewed at least once a year [2, 11, 14].
The reason is that a clot with no clear cause may come back after the medicine is stopped: in about 1 in 10 people within 2 years and in more than 3 in 10 within 10 years [2]. Never stop the medicine on your own; make this decision with your doctor.
For physicians
Choice of anticoagulant. ASH 2020 suggests DOACs over vitamin K antagonists (VKA) for DVT and PE (conditional recommendation, moderate certainty) and does not favour one DOAC over another [2]. CHEST 2021 also recommends apixaban, rivaroxaban, dabigatran or edoxaban over VKA for the treatment phase (the first 3 months) [12]. NICE considers apixaban or rivaroxaban first line; if neither is suitable, LMWH for at least 5 days followed by dabigatran or edoxaban, or LMWH with a VKA for at least 5 days or until the INR is at least 2 on two consecutive readings, followed by a VKA alone [11]. In triple-positive antiphospholipid syndrome, LMWH with a VKA followed by a VKA alone [11]; CHEST also prefers a VKA over a DOAC in antiphospholipid syndrome [12]. Doses, renal adjustment, reversal and interruption before surgery: Blood thinners (section for doctors).
Outpatient or inpatient; thrombolysis and filters. Home treatment is preferred over hospital admission for uncomplicated DVT and low-risk PE (conditional recommendation) [2]; NICE considers outpatient treatment of low-risk PE using a validated risk stratification tool and a clear follow-up plan [11]. In PE with haemodynamic compromise, thrombolysis followed by anticoagulation (strong recommendation) [2] and, according to NICE, a continuous UFH infusion [11]. In PE with right ventricular dysfunction but without haemodynamic compromise, and in most proximal DVTs, anticoagulation alone is preferred over thrombolysis [2]; NICE does not recommend systemic thrombolysis in haemodynamically stable PE, with or without right ventricular dysfunction [11]. An IVC filter should not be used except in a clinical study, when anticoagulation is contraindicated or after PE during anticoagulation (once the causes of treatment failure have been addressed), and a removal plan should be in place before it is inserted [11].
Compression stockings and walking. ASH suggests against the routine use of compression stockings, with or without a high risk of PTS (conditional recommendation, very low certainty); in selected patients they may reduce acute oedema and pain [2]. NICE also does not offer elastic stockings to prevent PTS or recurrence [11]. In a meta-analysis of 13 studies (3,269 patients), early walking with anticoagulation, compared with bed rest, was not associated with more new PE, DVT progression or DVT-related deaths, and in moderate to severe pain it was associated with better pain relief [13].
| Type of event | Definition or examples | After primary treatment |
|---|---|---|
| Provoked by a transient risk factor | A major, transient clinical risk factor in the past 3 months, such as surgery, trauma, significant immobility, pregnancy or the puerperium, or hormonal therapy (combined pill or menopausal hormone therapy) [11] | A shorter course of 3 to 6 months rather than 6 to 12 months (ASH) [2]; NICE: consider stopping after 3 months if the provoking factor is no longer present and the course has been uncomplicated [11] |
| Chronic risk factor | A risk factor that persists [2]; for example active cancer: invasive cancer diagnosed in the past 6 months (other than basal cell or squamous cell skin carcinoma), cancer treated in the past 6 months, or recurrent or metastatic cancer [14] | Indefinite antithrombotic therapy rather than stopping (ASH, conditional recommendation) [2]; in active cancer, long-term treatment beyond 6 months [14] |
| Unprovoked | No recent major clinical risk factor and no hormonal therapy [11] | Indefinite antithrombotic therapy rather than stopping (ASH, conditional recommendation) [2]; NICE: continue beyond 3 months after weighing the risks of recurrence and bleeding (for example HAS-BLED; with a score of 4 or more that cannot be modified, discuss stopping) [11] |
| Recurrent unprovoked VTE | A second event with no provoking factor | Indefinite antithrombotic therapy (ASH, strong recommendation) [2] |
In the extended phase, a DOAC at standard or reduced dose (ASH) [2]; CHEST prefers reduced-dose apixaban 2.5 mg twice daily or rivaroxaban 10 mg once daily [12]. Anticoagulation is preferred over aspirin, and the INR target with a VKA is 2 to 3 (strong recommendation) [2]. To decide the duration of treatment in unprovoked VTE, the routine use of prognostic scores, D-dimer or ultrasound for residual thrombosis is not suggested [2]. Risk of recurrence after stopping in unprovoked VTE: about 10% by 2 years and more than 30% by 10 years [2]. Review long-term treatment at least once a year [11].
Prevention
On a long journey (more than 4 hours) [1, 4]:
- Get up and walk around every 1 to 2 hours.
- Wear loose-fitting clothes.
- Exercise your legs while sitting: raise and lower your heels while keeping your toes on the floor, raise and lower your toes while keeping your heels on the floor, and tighten and relax your calf muscles.
- If you have had a clot before, have recently had surgery, are pregnant or have recently given birth, or have cancer, talk to your doctor before travelling about compression stockings or a preventive injection [4, 16].
After surgery or a hospital stay: get out of bed and walk as soon as possible, with your doctor’s agreement [1]. Most clots after surgery appear after leaving hospital, up to about 3 months later [15]. Many of these clots can be prevented with preventive medicine or compression stockings [15]; if you have been prescribed a medicine, finish the course and learn the signs of a clot.
In everyday life: a healthy weight and being physically active lower the risk [1]. If anyone in your family has had a clot, tell your doctor, especially before surgery or in pregnancy [3, 5].
For physicians
Long-distance travel of more than 4 hours (ASH 2018). In travellers without risk factors, compression stockings, LMWH or aspirin are not suggested. In people at substantially increased risk (recent surgery, previous VTE, postpartum, active cancer, or two or more risk factors including combinations of these with menopausal hormone therapy, obesity or pregnancy), compression stockings or prophylactic LMWH, and if neither is feasible, aspirin rather than no prophylaxis; all conditional recommendations with very low certainty [16].
Blood clots and cancer
Cancer and some of its treatments raise the risk of a clot [7]. The risk is higher in cancers of the pancreas, stomach, brain, lung, womb, ovary and kidney and in lymphoma and myeloma; hospital stays, surgery, chemotherapy, hormone therapy and catheters (such as a port) also raise the risk [7].
If you have cancer, learn the signs of a clot and tell your treatment team straight away about swelling of one leg or arm or new breathlessness [7]. Clots in people with cancer are also treated with an anticoagulant. The doctor chooses the medicine according to the type of cancer, the risk of bleeding and interactions with cancer medicines, and usually continues treatment for as long as the cancer is active [11, 14]. Read about caring for your catheter in Caring for a chemotherapy port or PICC line.
For physicians
Cancer-associated thrombosis (ASH 2021). Initial treatment (first 5 to 10 days): a DOAC (apixaban or rivaroxaban) or LMWH; if a DOAC is not used, LMWH over UFH and over fondaparinux [14]. Short-term treatment (3 to 6 months): a DOAC (apixaban, rivaroxaban or edoxaban) over LMWH and over a VKA, and if a DOAC is not used, LMWH over a VKA; all conditional recommendations [14]. In unresected intraluminal gastrointestinal or genitourinary cancers, use DOACs with caution (higher risk of bleeding) [14]. In active cancer, long-term treatment beyond 6 months rather than short-term treatment, and indefinite continuation with a DOAC or LMWH rather than stopping after a set period, are suggested [14]. CHEST 2021 also recommends an oral factor Xa inhibitor (apixaban, rivaroxaban, edoxaban) over LMWH [12].
Interactions between DOACs and cancer drugs: Drug interactions with oral anticancer drugs. Full review, prophylaxis in outpatients and special situations: Venous thromboembolism in cancer and the Khorana score tool.
When should I see a doctor urgently?
Call your local emergency number straight away if [1]:
- you suddenly feel short of breath
- you have chest pain that gets worse when you breathe in deeply or cough
- you cough up blood
- your heartbeat is fast or irregular, you feel light-headed or you have fainted
See a doctor as soon as possible, preferably the same day, if one leg or arm has new swelling, pain, warmth or redness, especially after surgery, a hospital stay or a long journey, in pregnancy or during cancer treatment [1, 3].
If you take a blood thinner, see the signs of bleeding in Blood thinners.
Frequently asked questions
Does a blood clot in the leg go away on its own?
Is a blood clot with no clear cause a sign of cancer?
Usually not. But a clot that has no clear cause is linked to a higher chance of a cancer that has not yet been diagnosed [11]. This is why the doctor reviews your medical history, does a full examination and checks basic blood tests. Further tests for cancer are only needed if there are symptoms or signs that point to it [11].
How long do I need to take a blood thinner?
Can I fly after a blood clot?
Talk to your doctor before you travel; people who have had a clot before are at higher risk on long journeys [4]. If you take a blood thinner, keep taking it as your doctor has told you, and on the journey walk every 1 to 2 hours and move your legs [1, 4]. Your doctor may also recommend compression stockings or a preventive injection [16].
How is a blood clot treated in pregnancy?
In pregnancy, the medicine of choice is a low-molecular-weight heparin injection (such as enoxaparin) given under the skin [5, 17]. If you take a blood thinner and are planning a pregnancy or have become pregnant, tell your doctor as soon as possible and read Blood thinners.
Do I need a test for an inherited clotting disorder (thrombophilia)?
Most people do not. When a clot had a clear cause such as surgery, this test is not recommended [11]. If the clot had no clear cause and the medicine is going to be stopped, the doctor may consider a test for antiphospholipid syndrome, and if a first-degree relative has also had a clot, a test for an inherited clotting disorder. Anticoagulants can change the results of these tests [11].
How long does the leg swelling last after a clot?
Scientific evidence
Show detailsHide
Key studies on blood clots in cancer
5 itemsApixaban for the Treatment of Venous Thromboembolism Associated with Cancer
Comparison of an Oral Factor Xa Inhibitor With Low Molecular Weight Heparin in Patients With Cancer With Venous Thromboembolism: Results of a Randomized Trial (SELECT-D)
Apixaban to Prevent Venous Thromboembolism in Patients with Cancer
Rivaroxaban for Thromboprophylaxis in High-Risk Ambulatory Patients with Cancer
Edoxaban for the Treatment of Cancer-Associated Venous Thromboembolism
Related links
References
17 referencesShow all referencesClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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- 6Benefits of combined hormonal contraceptives (CHCs) continue to outweigh risks – CHMP endorses PRAC recommendation
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- 8Data and Statistics on Venous Thromboembolism
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- 17American Society of Hematology 2018 guidelines for management of venous thromboembolism: venous thromboembolism in the context of pregnancy
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