Deep vein thrombosis

Deep vein thrombosis (DVT) is a blood clot in a deep vein, most often in the leg. It can cause swelling, pain, warmth and redness, usually in one leg, but sometimes causes few symptoms. The main danger is that part of the clot breaks off and travels to the lungs (pulmonary embolism). Risk rises with immobility, recent surgery or hospital stays, cancer, pregnancy and the weeks after birth, estrogen-containing medicines, a previous clot, and inherited or acquired clotting disorders.

Lab tests linked to deep vein thrombosis:

• Cardiolipin antibody (IgM) and PTT-LA screen: tests for antiphospholipid syndrome, an acquired condition that raises clot risk. They are used in selected people, not to diagnose a clot.
• DVT is diagnosed with a leg ultrasound. A D-dimer blood test is often used first when the chance of a clot is low. Doctors also check a CBC, kidney function and clotting tests before treatment.

How results are read:

• A normal D-dimer, in someone at low risk, makes a DVT unlikely. A raised D-dimer is common with age, pregnancy, surgery, infection and cancer, so it doesn't confirm a clot.
• Antiphospholipid testing is usually reserved for selected cases, such as clots without a clear trigger in younger people. Blood thinners and acute illness can affect results.
• Repeat testing at least 12 weeks apart checks whether antiphospholipid antibodies persist. A diagnosis of antiphospholipid syndrome also depends on the specific antibody results and the clotting or pregnancy history.

When to see a doctor:

• Get same-day urgent assessment for a swollen, painful leg, especially after surgery, travel or a period of immobility.
• Call your emergency number for sudden breathlessness, chest pain, coughing up blood or fainting.

Clinical takeaway: DVT is diagnosed with ultrasound, often after a D-dimer test, and antiphospholipid tests are used only in selected people to look for an underlying cause.