Thrombophilia and blood clotting disorders explained

What thrombophilia means

Thrombophilia is not a single disease but a group of conditions that make the blood more likely to clot inside a vein. Normally the body keeps a careful balance between clotting, which stops bleeding, and keeping blood flowing freely. In thrombophilia that balance leans towards clotting. This raises the risk of a deep vein thrombosis, a clot usually in a leg, and of a pulmonary embolism, where a clot travels to the lung. It is important to keep the risk in proportion: many people with a mild thrombophilia never have a clot at all, while a clot usually needs a second trigger — such as surgery, immobility, pregnancy or the contraceptive pill — on top of the underlying tendency.

Inherited thrombophilias

Some thrombophilias are inherited, caused by changes in the genes that control clotting. The most common in the UK is a variant called factor V Leiden, which makes the blood slightly more prone to clot; another involves the prothrombin gene. These are relatively common and usually cause only a mild increase in risk, so most people who carry them never have a clot. Rarer inherited conditions, such as deficiencies of the natural anticoagulant proteins the body makes, cause a stronger tendency to clot. Because these traits run in families, a personal or family history of clots at a young age, or in unusual places, may prompt a doctor to consider whether an inherited thrombophilia is present.

Acquired clotting disorders

Not all clotting problems are inherited. Some develop during life. The most important acquired thrombophilia is antiphospholipid syndrome, an immune condition in which the body makes antibodies that increase clotting; it can cause clots in veins or arteries and is also linked to pregnancy complications, including recurrent miscarriage. Other situations temporarily raise clotting risk without being lifelong disorders — for example pregnancy, cancer, some hormone treatments, prolonged immobility, and serious illness. Recognising these acquired causes matters, because treating or managing the underlying situation is part of reducing the risk. Antiphospholipid syndrome in particular is important to diagnose, as it changes how clots are treated and how pregnancies are looked after.

Who is tested, and who is not

Testing for thrombophilia is not done routinely, and often it is not helpful, because a positive result frequently does not change what treatment someone needs. UK guidance advises against testing most people who have had a clot triggered by a clear, temporary cause such as surgery. Testing may be considered in particular situations — for example certain unprovoked clots, clots at a young age or in unusual sites, a strong family history, or recurrent pregnancy loss where antiphospholipid syndrome is suspected. Testing is best arranged and interpreted by a specialist, often away from the time of an acute clot and off blood-thinning treatment, because these can distort the results. A careful decision about who to test avoids anxiety and unnecessary labelling.

Managing the risk of clots

Managing thrombophilia is about managing risk, not curing a fixed disease. Many people need no ongoing treatment and simply take extra care at high-risk times: for example, being offered blood-thinning injections around surgery, during hospital stays, or in pregnancy, and staying mobile and hydrated on long journeys. Someone who has already had a clot may be advised to stay on anticoagulant (blood-thinning) medication longer, especially with a stronger thrombophilia or an unprovoked clot. People with antiphospholipid syndrome usually need specific long-term anticoagulation. Lifestyle steps that lower general clot risk — not smoking, keeping active, maintaining a healthy weight — help too. Decisions balance the risk of clots against the risk of bleeding from treatment, so they are tailored to the individual.