Blood clots: symptoms, diagnosis and treatment explained

Your blood’s clotting ability is important to help stop excessive and potentially dangerous bleeding when a blood vessel is injured. However, blood clots can form inappropriately and cause a thrombus, where the clot reduces blood flow through a vessel — this can be potentially life-threatening. 

When blood clots don’t break down, they can lead to serious medical conditions. They can form in blood vessels in any part of your body, causing pain and swelling, as well as serious conditions such as heart attacks and stroke. 

Understanding what blood clots are and which symptoms to look out for can help keep you and others safe in the event that a harmful blood clot forms.

What are blood clots?

A blood clot is blood that has changed from its liquid state to a semi-solid or gel-like clump. Normally, a blood clot would break up and dissolve as part of the healing process, however, this doesn’t always happen.

When a clot forms in a vein or artery, it doesn’t always break up. While a blood clot that doesn’t move won’t usually cause harm, there is a chance that it will break free and travel through your blood vessels. Once the clot is in your veins, it can travel to your lungs or your heart, where it can get stuck and stop blood flow.

When a clot forms in an artery, which carries oxygen in your blood to the cells of your body, it can stop oxygen from getting to your brain, heart or lungs. This is also life-threatening and can cause a stroke or heart attack.

How do you know if you have a harmful blood clot?

As harmful blood clots can affect different parts of your body, the signs that you may have one can vary.

Blood clots in your legs and arms

Blood clots are most likely to form in your legs, especially if you’re sitting down for long periods, such as when you’re driving long distances, taking a flight or have had surgery. However, they can also form in your arms. In either case, they tend to form in deep veins, which is why they’re referred to as a deep vein thrombosis (DVT).

DVT can result in a blood clot travelling to your heart or your lungs; in your lungs it can cause a pulmonary embolism (PE), which is life-threatening. If you have signs of a DVT, seek urgent medical attention.

DVT symptoms include:

• Colour change — your arm or leg may take on a red or blue tinge
• Cramps in your leg or arm
• Pain or itching — you may experience a dull ache, throbbing or intense pain in your arm or leg, which may get worse; you may also feel itchy
• Pitting oedema — a sudden build-up of fluid in your arm or leg, which when pressed, forms a dimple or pit that stays for several seconds
• Swelling where the clot has formed or across your entire leg or arm — if your veins swell they may become more prominent and painful, with the pain increasing when you touch them

If you experience trouble breathing it may be a sign that the clot has moved from your arm or leg to your lungs, causing a pulmonary embolism — you should get medical attention immediately.

Blood clots in your heart

If a blood clot forms in or around your heart, or if one travels to your heart, it can lead to a heart attack. Signs of a heart attack include:

• Difficulty breathing
• Severe pain in your chest and left arm
• Sweating

If you experience these symptoms, call 999 or go straight to A&E.

Blood clots in your lungs

In most cases, blood clots in your lungs have travelled there from your arm or leg. As mentioned earlier, this is called a pulmonary embolism (PE) and is a medical emergency.

Signs of a pulmonary embolism include:

• Breathlessness
• Chest pain
• Coughing
• Dizziness
• Sweating

Blood clots in your brain

Blood clots in your brain can occur for several reasons, including a head injury or blow to your head, as well as fatty deposits in the walls of the blood vessels that supply your brain. They can also travel to your brain from other parts of your body. Blood clots in your brain can cause a stroke.

Signs of a blood clot in your brain include:

• A seizure
• Muscle weakness
• Speech problems
• Vision problems

Blood clots in your intestines

Blood clots can also form in the veins that drain blood from your intestines. This type of blood clot can be caused by certain medications, such as the contraceptive pill, or as a result of medical conditions such as liver disease or diverticulitis. 

You should talk to your GP as soon as possible if you experience any of these symptoms:

• Bloating
• Bloody stools
• Diarrhoea
• Nausea
• Severe stomach pain, which may worsen after eating
• Vomiting

Blood clots in your kidneys

Your kidneys remove waste from your body and blood clots can stop them from doing this. The result can be kidney failure – if you experience any of the following symptoms you should seek urgent medical attention:

• Blood in your urine
• Difficulty breathing
• Fever, nausea or vomiting
• High blood pressure
• Pain in the side of your stomach, thighs or legs
• Sudden leg swelling

How are blood clots diagnosed and treated?

As well as a physical examination, your doctor may send you for a blood clot test. This may include: 

• CT angiography of the chest, abdomen and pelvis, head and/or neck
• Doppler ultrasound scan
• Magnetic resonance angiography (MRA)
• Venous ultrasound
• Venography

As blood clots can cause symptoms associated with other conditions, you may also have other tests to rule out other causes. 

If you’re diagnosed with a blood clot, the type of treatment will depend on the type of clot you have.

Arterial blood clots

With this type of clot, you may need catheter-directed thrombolysis. This is where medications are delivered to the site of the clot to break it up. Alternatively, surgery may be needed to remove the clot.

As arterial clots can block blood flow to vital organs, emergency treatment is often needed.

Venous clots

If you have deep venous clots ie blood clots in your veins, you’ll likely be given blood-thinning medication. This will prevent the blood clot from getting larger, while your body breaks it down and will prevent more blood clots from forming.

If you’re at high risk of this type of clot, you may need to have a procedure called an inferior vena cava filter placement. This is when a filter is placed in your vein to stop clots from travelling to your heart or lungs.

Who is at risk from blood clots?

While blood clots can affect anyone, there are several risk factors that may increase your risk:

• Being overweight
• Having previously had a blood clot
• Inflammatory conditions eg rheumatoid arthritis or Crohn’s disease
• Pregnancy or having recently given birth
• Smoking
• Taking combined hormonal contraception (the pill, contraceptive patch or vaginal ring)

You’re also more likely to develop a blood clot after a hospital stay.