One in four deaths worldwide is linked to conditions caused by thrombosis, according to the International Society on Thrombosis and Haemostasis (ISTH). Many families only hear the word after a relative develops a swollen leg following surgery, or struggles to breathe after a long hospital stay.
That gap in awareness matters for patients, caregivers and clinicians alike. Some clots cause no symptoms until they reach the lungs, and by then the situation can be life-threatening.
Thrombosis is the formation of a blood clot, called a thrombus, inside a blood vessel, where it slows or blocks normal blood flow. Many cases can be prevented, and most can be treated when they are recognised early. World Thrombosis Day, held every year on 13 October, exists to make early recognition more common.
In this guide, you’ll learn what thrombosis is, who is most at risk, which warning signs need action, and how D-dimer testing supports diagnosis.
What Is Thrombosis?
Clotting is protective. When a vessel is injured, the body forms a clot to stop bleeding. Thrombosis occurs when a clot forms inside a vein or artery without that need.
Venous thrombosis affects the veins. Its best-known form is deep vein thrombosis (DVT), a clot in a deep vein, usually in the leg. If part of that clot breaks away and lodges in the lungs, it causes a pulmonary embolism (PE). The CDC groups DVT and PE together as venous thromboembolism (VTE).
Arterial thrombosis affects the arteries. A clot in an artery supplying the heart can cause a heart attack. A clot that blocks blood flow to the brain can cause an ischaemic stroke.
Why Do Blood Clots Form?
Three processes drive clot formation: blood that flows slowly or pools, damage to a vessel wall, and blood that clots more readily than normal. Prolonged bed rest slows circulation. Surgery and injury damage vessels. Pregnancy and raised oestrogen levels increase the blood’s tendency to clot.
The CDC lists the main risk factors for DVT and PE as hospitalisation, major surgery, prolonged immobility, certain cancers, pregnancy and the period after childbirth, a previous clot, inherited clotting disorders, older age and obesity. Risk rises when several of these factors occur together.
Hospitals carry a large share of this risk. Patients who are acutely ill, confined to bed or recovering from an operation are among those most likely to develop VTE. This is why risk assessment on admission is central to prevention.
What Are the Warning Signs of a Blood Clot?
Some people with DVT have no symptoms at all. When symptoms do appear, they usually affect one limb. The leg may swell or feel painful or tender without an obvious injury. The area may also feel warm and look red or discoloured.
A pulmonary embolism presents differently. Warning signs include sudden shortness of breath and chest pain that worsens with a deep breath or cough. The heartbeat may be fast or irregular, the person may cough up blood, and severe cases can cause light-headedness or fainting. PE is a medical emergency, and anyone with these symptoms needs urgent hospital care.
How Does a D-Dimer Test Help Diagnose Thrombosis?
D-dimer is a protein fragment released into the blood when the body breaks down a clot. A D-dimer test measures how much of it is present.
A raised result does not confirm thrombosis. D-dimer levels also rise after surgery, during pregnancy, with infection, inflammation or cancer, and with age. Clinicians therefore interpret the result alongside the patient’s symptoms, history and an assessment of how likely a clot is.
The test is most useful for ruling a clot out. American Society of Hematology (ASH) guidelines support using a negative D-dimer to exclude DVT or PE in patients with a low or intermediate clinical probability of VTE. When suspicion remains, clinicians move to imaging, such as ultrasound of the leg or a CT scan of the lungs.
D-dimer answers one question well: does this patient need further investigation for a clot? For hospitals, clinics and laboratories, reliable D-dimer testing shortens the time it takes to reach that answer.
Why Does Early Recognition Matter?
The outcome of thrombosis depends on where the clot forms and whether it travels. An untreated DVT can progress to a PE. An arterial clot can cut off blood supply to the heart or brain.
Healthcare professionals lower this risk by assessing patients on admission and after surgery. They then match prevention to each patient’s risk, using measures such as early mobilisation, compression devices or anticoagulant medicines.
Know the Risk. Recognise the Signs.
Thrombosis is common, often preventable and too often missed. Unexplained swelling or pain in one leg deserves a medical review. Sudden breathlessness, chest pain or coughing up blood needs emergency care.
If you have any of these risk factors or symptoms, speak to your doctor. If your hospital, clinic or laboratory is reviewing its coagulation testing, ISN Medical can help.