Blood Clots: Signs, Causes, When to Worry and Which Tests Find Them
The essentials
Blood clots form naturally to stop bleeding, but a clot inside a vein or artery can block blood flow. A swollen, painful, warm leg may mean a deep vein clot, and sudden chest pain or trouble breathing may mean a clot in the lung. Sudden weakness, speech trouble or facial droop may mean a stroke. These are emergencies. A D-dimer blood test can help rule a clot out, but a high result is not a diagnosis and imaging usually confirms it.
When should I get urgent help for a possible blood clot?
When to see a doctor
- Emergency: call 911 for sudden chest pain, trouble breathing, a rapid heartbeat or coughing with chest pain, which can signal a clot in the lung[4]. Also call for sudden confusion, trouble speaking, numbness or weakness on one side, sudden severe headache or sudden loss of vision, which are signs of stroke. Do not drive yourself; an ambulance can start treatment on the way[5].
- Within hours to 24 hours for a leg that is swollen, painful or tender, red or darker, and warm to the touch, especially after surgery, injury or a long period without moving[4][2]. Call your health care provider right away; deep vein thrombosis can lead to a life-threatening lung clot[2].
- At your next appointment to talk about your clot risk if you have a family history, are scheduled for major surgery or have several risk factors[1][3].
Sometimes a clot causes no symptoms until serious complications develop[1]. If you have risk factors and any of these signs, do not wait for them to go away.
What are the signs of a blood clot?
- Leg or arm (deep vein thrombosis). Pain or tenderness, swelling, redness or darker skin, and skin that feels warm. It most often occurs in the lower leg, thigh or pelvis, and can also occur in an arm, especially with a central line[4][1].
- Lung (pulmonary embolism). Trouble breathing, cough, chest pain, and a rapid heartbeat or fast breathing. Many people have no symptoms of pulmonary embolism[4].
- Brain (stroke). Face drooping, arm drifting down, slurred speech: act FAST and call 911[5].
Clots in a period or in stool are a different matter. Dark or black stool, for example, is covered in our guide to black stool.
What causes blood clots, and who is at risk?
Clots can form when blood flow slows or the lining of a vessel is damaged, and most people with a venous clot have more than one risk factor[3]. As many as 600,000 venous clot events occur each year in the United States[1].
- Common Not moving: bed rest, hospital stays, after surgery (risk is highest in the first 3 months) and flights longer than 4 hours[3].
- Common Recent surgery, injury or serious infection, including COVID-19[1][3].
- Common Age: the chance nearly doubles every 10 years after age 40[3].
- Possible Medical conditions: cancer and its treatment, diabetes, heart attack or heart failure, high blood pressure, kidney disease, obesity and spinal cord injury[3].
- Possible Family history of clots: siblings of someone who has had one have more than double the likelihood[3].
- Rare Inherited or acquired clotting disorders, sometimes found only after a first clot[3].
Which tests can find a blood clot?
Your provider looks at your history, risk factors and exam, then orders blood and imaging tests[2].
| Test | What it shows | Good to know |
|---|---|---|
| D-dimer (blood) | A fragment released when a clot dissolves[4] | Can help rule out a clot in people at low risk; it is not specific[2][4] |
| Doppler ultrasound | Blood flow in leg or arm veins[4] | Imaging test used when a vein clot is suspected[4] |
| CT pulmonary angiography | Clots in the lung vessels | The main test to diagnose a lung clot[2] |
| Ventilation-perfusion (V/Q) scan | Air and blood flow in the lungs | Alternative to CT angiography[2] |
| Blood oxygen level | How well the lungs are working | Low oxygen can be a sign of a clot[2] |
Other blood tests, such as prothrombin time, platelet count and fibrinogen, describe how your blood clots, and your doctor may add them if you have repeated clots or take blood thinners.
How do I read a D-dimer result?
D-dimer is a protein fragment made when a blood clot dissolves. It is not usually found in the blood unless the body is making or breaking up clots[4]. A normal or low level means a clotting disorder is probably not present, but blood thinners can give a false negative[4]. A high level may signal a clot, but it also rises with pregnancy, heart disease, rheumatoid arthritis, recent surgery, older age and immobility[4]. Here is a fictitious report.
| Test | Result | Reference range | Flag |
|---|---|---|---|
| D-dimer, quantitative | 820 ng/mL FEU | Less than 500 ng/mL FEU | H |
| Platelet count | 245 x10^3/µL | 150-400 x10^3/µL | - |
| INR | 1.0 | 0.8-1.2 | - |
- Result and unit: D-dimer can be printed in FEU or DDU units, which are not interchangeable; compare only with your own lab's range.
- Flag H: above the range. This alone does not prove a clot; your provider combines it with symptoms and risk before choosing imaging.
- Age-adjusted cut-off: after age 50, many hospitals use age multiplied by 10 µg/L as the limit, to reduce unnecessary scans[6].
Have a D-dimer or clotting result flagged H? BloodSense explains each line of your report and helps you prepare questions for your doctor.
Get my results explainedWhat should I note before seeing a doctor?
- When the swelling, pain or breathlessness started, and which side.
- Recent surgery, injury, hospital stay, long trip or time in bed.
- Past clots, and clots in close relatives.
- Every medicine and supplement, including blood thinners and hormones.
- Any cancer diagnosis or treatment, and any recent infection.
Questions to ask your doctor
- Is my risk of a blood clot higher than usual, and why?
- Do I need a D-dimer test, an ultrasound or a CT scan?
- How do I tell a clot from a pulled muscle or an infection in the leg?
- Are any of my medicines raising my risk?
- How can I lower my risk on long trips or after surgery?
Frequently asked questions
Can a normal D-dimer rule out a blood clot?
A normal or low D-dimer means you probably do not have a clotting disorder, but blood thinners can cause a false negative. Your provider also weighs your symptoms and risk[4].
Can a blood clot go away on its own?
The body does dissolve clots after an injury has healed[4], but a clot in a deep vein can break loose and travel to the lungs, so it needs medical assessment[1].
Can a clot cause no symptoms at all?
Yes. A venous clot sometimes shows no signs until serious complications develop[1].
How can I lower my risk of clots?
Ask about prevention before major surgery, move regularly on long trips and during hospital stays, and treat conditions such as diabetes and high blood pressure[3]. Your provider can advise on medicines to prevent clots if you are at high risk[1].
Related articles
- Deep vein thrombosisThe most common clot in the leg: symptoms, causes and treatment
- Pulmonary embolismWhen a clot travels to the lungs
- D-dimer testHow to read the units and the cut-off
- StrokeWarning signs and what to do
- Prothrombin time (INR)A test of how fast your blood clots
- National Heart, Lung, and Blood Institute. Venous Thromboembolism. National Institutes of Health. nhlbi.nih.gov (accessed October 23, 2026)
- National Heart, Lung, and Blood Institute. Venous Thromboembolism: Diagnosis. nhlbi.nih.gov (accessed October 23, 2026)
- National Heart, Lung, and Blood Institute. Venous Thromboembolism: Causes and Risk Factors. nhlbi.nih.gov (accessed October 23, 2026)
- MedlinePlus. D-Dimer Test. US National Library of Medicine. medlineplus.gov (accessed October 23, 2026)
- National Heart, Lung, and Blood Institute. Stroke: Symptoms. nhlbi.nih.gov (accessed October 23, 2026)
- Konstantinides SV, Meyer G, Becattini C, et al. 2019 ESC guidelines for the diagnosis and management of acute pulmonary embolism. Eur Heart J. 2020;41(4):543-603. doi:10.1093/eurheartj/ehz405 (accessed October 23, 2026)
This page is general information. It does not replace the advice of a doctor, who interprets your results with your symptoms, history and examination.
