What is thrombosis? Everything you need to know about blood clots in the legs

Thrombosis refers to the formation of a blood clot that blocks or partially blocks the flow of blood in a vein or artery. When this occurs in the deep veins of the legs, it is called deep vein thrombosis (DVT). This is a serious condition because a clot can sometimes dislodge and travel to the lungs, causing a pulmonary embolism.

Many people associate blood clots with heart problems, but venous blood clots frequently occur in the legs. This is largely because the blood has a longer journey back to the heart against gravity, especially if the vein valves are weakened or damaged — a common issue in people with varicose veins. You can learn more about this connection by reading facts about varicose veins.

Thrombosis symptoms: how to recognise them

Recognising thrombosis symptoms early is crucial because they often appear suddenly and usually affect only one leg. Common signs include:

• Swelling — often affecting the entire leg
• Pain or tenderness, typically in the calf
• Redness or discolouration of the skin
• Warmth when touching the affected area
• A feeling of heaviness or tightness in the leg

It is easy to confuse these symptoms with muscle cramps or fatigue. However, the intensity and persistence of symptoms are key indicators that something more serious may be happening. For more insight into other causes of leg pain, you might find it helpful to read aching legs – causes and treatments.

Why does thrombosis occur?

Thrombosis typically results from one or more elements of Virchow's triad: sluggish blood flow, changes in the blood's makeup, or damage to the blood vessel wall. Several factors can increase the risk:

• Long periods of immobility, such as during long-haul flights or hospital stays
• Pregnancy or hormone therapy
• Smoking
• Being overweight
• A family history of thrombosis
• Varicose veins and chronic venous insufficiency

In fact, people with venous insufficiency are at a particularly elevated risk. If you want to understand more about poor blood circulation and vein health, our guide on venous insufficiency offers useful insights.

Diagnosing and treating thrombosis

If you suspect you might have thrombosis, seek medical attention immediately. Diagnosis is usually confirmed with an ultrasound scan, while a D-dimer blood test can help detect abnormal clotting activity but cannot rule out a clot by itself.

Treatment typically involves taking blood-thinning medication for three to six months. Compression stockings are often prescribed to promote better blood flow and prevent long-term complications like post-thrombotic syndrome. To explore treatments in more depth, you can read deep vein thrombosis: symptoms, causes and treatment.

The link between varicose veins and thrombosis

Many people are unaware that having varicose veins can increase the risk of thrombosis. When vein valves fail, blood pools in the veins, making clot formation more likely.

If you have varicose veins and notice new symptoms like swelling, redness or warmth in one leg, it is important to have it assessed. Understanding how to manage varicose veins early on can help prevent complications — our guide to how to avoid varicose veins offers some practical advice.

How to tell the difference: thrombosis or tired legs?

To help you differentiate between normal fatigue and a more serious problem like thrombosis, we’ve created a comparison table below:

When in doubt, it’s always safest to consult a healthcare professional.

Protect your legs – act early if you notice signs of thrombosis

Recognising the symptoms of thrombosis early can make a life-saving difference. Ignoring swelling, pain or warmth in one leg could allow a clot to grow or travel, leading to serious complications like pulmonary embolism.

At UK Vein Care, we often see patients who delay seeking help. Getting assessed early is key to avoiding future issues. If you are experiencing concerning symptoms, don't hesitate to reach out for advice.