A deep vein thrombosis is a blood clot in one of the deep veins, usually in the leg. It matters for two reasons: a piece can break off and travel to the lungs, and about half of DVTs cause no symptoms at all. No symptom list can rule one out.
Call 911 now if you have any of these
- Difficulty breathing that came on suddenly
- Chest pain or discomfort, usually worse with a deep breath or coughing
- Coughing up blood
- A faster than normal or irregular heartbeat
- Lightheadedness, fainting, or very low blood pressure
These can be signs of a pulmonary embolism — a clot that has travelled to the lungs. This is an emergency. Outside the United States, use your local emergency number.
Be seen today if you have these
Swelling, pain or tenderness, warmth, or redness or discoloration in one leg — particularly when the other leg is normal, and particularly if it developed over hours to days. You need an assessment and usually an ultrasound, today rather than at the next available appointment.
Why timing is the whole story
A leg clot is treatable. Anticoagulation stops it growing and lets the body break it down, and it substantially reduces the chance of a piece travelling to the lungs. The reason urgency matters is not that the leg will be lost — it is that the lung complication is the dangerous one, and the window to prevent it is before it happens.
There is a second, slower reason. A clot damages the vein valves it sits against. That damage drives post-thrombotic syndrome — chronic swelling, aching and skin change in that leg, sometimes years later. Prompt treatment reduces that too.
What raises the risk
- Recent surgery, particularly hip, knee or abdominal, and recent major trauma
- Prolonged immobility — hospital admission, a plaster cast, a long flight or drive
- Cancer and some cancer treatments
- Pregnancy and the weeks after delivery
- Oestrogen-containing contraception and hormone therapy
- A previous clot, or a family history of clots
- Inherited or acquired clotting disorders
- Obesity, older age, and smoking
- Central venous catheters
Risk factors raise the probability; their absence does not remove it. Plenty of clots happen in people with none of these.
Why we will not give you a checklist that rules it out
Because none exists. Around half of deep vein clots produce no symptoms. Of those that do, the symptoms — swelling, ache, warmth, redness — overlap almost completely with a pulled muscle, a ruptured Baker’s cyst, cellulitis and a flare of chronic vein disease. Clinicians do not exclude DVT by examination either; they use a structured risk assessment, a blood test and an ultrasound. That is the honest answer, and it is why the advice here is simply to be seen.
How it is diagnosed
- A structured clinical assessment combining symptoms and risk factors into a probability.
- A D-dimer blood test in lower-probability situations. A negative result in that setting is reassuring; a positive one is non-specific and leads to imaging.
- Compression ultrasound of the leg veins — the main test. If the vein does not compress under the probe, there is clot in it.
- Imaging of the chest — usually CT pulmonary angiography — if pulmonary embolism is suspected.
Note that this is a venous ultrasound. It is a different study from an arterial ultrasound, and “I had a leg ultrasound” does not tell you which question was asked. More on the two ultrasounds →
Treatment, in outline
Anticoagulation is the mainstay — direct oral anticoagulants for most people, sometimes heparin and warfarin depending on circumstances such as pregnancy, cancer or kidney function. Duration depends on why the clot happened: a clot provoked by surgery is usually treated for a defined period, while an unprovoked clot or a recurrent one may mean longer or indefinite treatment. Compression may be used for symptoms afterwards, and clot-removal procedures are reserved for specific severe presentations.
These decisions turn on details of your own history and bleeding risk. This page is a map, not a plan.
Common questions
Can I tell a DVT from a pulled muscle?
Not reliably, and neither can a clinician without testing. A muscle injury usually has a story — a specific movement, a moment it happened. A clot often does not. But plenty of clots are blamed on an imagined strain, which is exactly how they get missed. One newly swollen or painful calf with no clear injury is a reason to be checked.
Should I massage my calf or stretch it out?
No. If a clot is a possibility, leave the leg alone and get it assessed rather than manipulating it. Get the ultrasound first and the physiotherapy afterwards if it turns out to be a muscle.
I have no risk factors. Can I relax?
No. A substantial share of clots occur in people with no identifiable risk factor. The absence of risk factors lowers the probability; it does not make the ultrasound unnecessary when the leg looks like this.
Does a clot mean I will always have a swollen leg?
Not necessarily, but post-thrombotic syndrome is a real possibility — persistent swelling, aching and skin change in that leg, developing over months to years. Taking the anticoagulation as prescribed and following the advice you are given about compression and activity reduces the chance.
Can I fly after a DVT?
Usually yes, once treatment is established, but the timing and precautions are individual. Ask the team treating you rather than working from general travel advice — this is one of the questions where your specific situation genuinely changes the answer.
Where to go next
Sources
- Centers for Disease Control and Prevention. About Venous Thromboembolism (Blood Clots).
- National Heart, Lung, and Blood Institute. Venous Thromboembolism.
- NHS. Deep vein thrombosis (DVT).
- NHS. Blood clots.
Written by Tom Biernacki, DPM, FACFAS. Published 20 September 2026. General education, not medical advice. This page has not yet been through the independent clinical review described on our medical review page.