Blood clot (thrombosis)

Last updated:

Reviewed by:

Dr Rhianna McClymont

, Lead GP at Livi

Medically reviewed

A blood clot consists of congealed blood that blocks the flow of blood in a vessel somewhere in the body. They often form in the legs, but can also occur, for example, in blood vessels that supply the lungs or heart. Blood-thinning medicines are used to prevent and treat blood clots. You should always get medical help, regardless of where a blood clot may form, as large clots can be life-threatening.

Call 112 or 999 if you or anyone near you has sudden onset shortness of breath, chest pains or signs of a stroke, which is a blood clot in the brain.

What are blood clots?

Blood clots are called thromboses in medical terminology, stemming from the Latin word for blood platelets, called thrombocytes. Platelets help the blood to clot or stiffen.

The ability of the blood to clot is vital to the body – it prevents us from bleeding to death if a blood vessel is damaged. But if the blood clots inside a blood vessel, a blood clot can form that may obstruct or even stop the blood flow to parts of the body, causing hypoxia (a lack of oxygen).

Sometimes part of a blood vessel can break off and move from one part of the body to another. This is called an embolism. For example, blood clots often occur in the deep veins of the calf. Sometimes part of a blood clot present here may break off and travel to the lungs where it blocks blood flow and thus oxygen supply. This is called a pulmonary (lung) embolism. When a blood clot forms in the heart and moves to the vessels supplying the brain, it causes a stroke.

Symptoms of blood clots

There are a range of symptoms which are dependent on the size and location of the blood clot.

Common symptoms of blood clots in the leg (known as deep vein thrombosis):

  • A warm feeling in the leg

  • Swelling in the calf or whole leg

  • Redness

  • Soreness and pain in the calf or whole leg

Common symptoms of blood clots in the lungs (known as a pulmonary embolism):

  • Shortness of breath and/or difficulty breathing

  • Heart palpitations and/or fast heart rate

  • Sudden chest pains that get worse when breathing deeply or coughing

  • Coughing, sometimes with coughing up blood

  • Dizziness, sometimes with fainting or collapse

Common symptoms of a blood clot in the brain (a stroke):

  • Numbness or altered sensation, often in one half of the face, one arm or leg

  • Weakness or paralysis in one arm or leg

  • Difficulty speaking or slurred speech

  • Loss of vision

  • Problems with coordination

  • Dizziness and problems with balance

  • Unusual behaviour and confusion

Blood clots can occur elsewhere, including the kidneys, liver and intestines. They can also occur in connection with serious illnesses.

Other possible explanations

The symptoms typically found with a blood clot can also be caused by other medical problems.

In the leg, other problems to consider include:

  • Muscular strain or arthritis, which can cause swelling and aches. Varicose veins
    can also make the leg feel tight or heavy, especially with cramps or itching.

  • Heart failure and lymphedema can also cause swelling in both legs

  • Baker's cyst can cause swelling in one leg, often starting in the crook of the knee

In the lungs, persistent chest pains with difficulty breathing and rapid pulse can be signs of other diseases involving the heart, blood vessels or lungs – like a heart attack (myocardial infarction), heart failure or inflammation of the heart muscle (myocarditis). Asthma, COPD and other respiratory problems can also cause shortness of breath and breathing difficulties.

The symptoms of a blood clot in the brain can resemble other diseases, like loss of vision due to migraine. Confusion and difficulty with coordination can also be signs of a brain tumour.

What causes blood clots?

Blood clots are caused by congealed blood that obstructs the normal flow of blood in the body's blood vessels. It then becomes harder for the heart to pump blood, and the blood is not as oxygen-rich as it should be. This can lead to the vital organs getting insufficient oxygen.

Risk factors that increase the likelihood of blood clots:

  • Advanced age – particularly those over 60

  • Genetics – a history of blood clots in the family, or a personal history of a condition which makes your blood more likely to clot

  • A history of previous blood clots

  • Other medical conditions like cardiovascular diseases, diabetes, cancer or varicose veins

  • Immobility – for example, after recent surgery, or on a long-haul flight

  • Oestrogen treatment, such as contraceptive pills and other oestrogen preparations

  • Pregnancy and the first 6 weeks after giving birth

  • Lifestyle factors like obesity, smoking, high alcohol consumption and insufficient physical activity

Vaccines and medications

During the outbreak of covid-19, a slightly higher risk of blood clots and haemorrhaging (bleeding) has been reported in connection with certain vaccines against covid-19. There are studies that indicate that certain vaccines in exceptional circumstances can affect the ability of the blood to clot. There are also other medications that can cause an increased risk of blood clots, like those in connection with treatment for serious illnesses.

Diagnosis and treatment

If you suspect a blood clot you should always book an urgent appointment with a doctor. A blood clot is diagnosed through a combination of your symptoms and examination findings, blood tests to assess your coagulation status, and scans such as an ultrasound scan or a venogram to assess for blood flow and blockages.

If a pulmonary embolism is suspected you may be offered a special CT scan of your lungs (a CTPA) to look for blood flow in the pulmonary arteries in the lungs, or a ventilation-perfusion scan which uses a safe radioactive gas to assess the flow of air and blood in your lungs.

To prevent and treat blood clots, you may often be given anticoagulants, either in tablet form or injections. They prevent the blood from coagulating too easily, and can dissolve existing blood clots and reduce the risk of new ones occurring.

In cases of severe blood clots an emergency procedure called a thrombectomy may be necessary. This involves inserting a small catheter into the artery which can be used to remove the clot and restore blood flow to the brain.

What can I do myself to prevent a blood clot?

Live a healthy lifestyle by maintaining a normal weight, not smoking and exercising regularly. Prevent dehydration by drinking plenty of fluids, and reduce your alcohol consumption.

Compression socks – available from pharmacies – can be useful for preventing blood clots if you know you’ll be immobile for a significant period of time.

When should I get medical help?

If you suspect a blood clot in the leg you should seek medical advice for further investigation.

Call 999 or 112 and ask for an ambulance in the event of the following symptoms:

  • Acute chest pain

  • Acute respiratory difficulties

  • Difficulty speaking or blurred vision

  • Numbness and paralysis

If you’re taking anticoagulants you must be aware there’s a higher risk of bleeding as your blood will not clot as it normally would. If you have an injury that makes you bleed, you must always tell healthcare personnel treating you that you’re on anticoagulant medicine.

How Livi can help

Always seek emergency care initially if you suspect a blood clot – particularly a blood clot in the brain or chest – as emergency treatment as quickly as possible is required.

If you’re unsure about your symptoms Livi doctors can discuss them with you and advise you on the best place to seek care.

Last updated:
Reviewed by:
Lead GP at Livi Dr Rhianna McClymont
Dr Rhianna McClymont, Lead GP at Livi