Blood tests for blood clots

Clinicians may use a D-dimer test when a clot is being considered. A low result can make an active clot less likely in selected people, but it is not suitable for every situation. Infection, inflammation, pregnancy, recent surgery, injury and other conditions can affect the result. A raised result does not prove a clot; ultrasound or a scan of the lungs may be needed, depending on symptoms and the suspected location. Blood tests can also check blood counts, kidney function or clotting before treatment.

Describe when symptoms began, whether one limb is swollen or painful, recent travel or immobility, surgery, pregnancy, cancer, previous clots and medicines. Do not massage a swollen painful leg or start someone else’s anticoagulant. Follow the assessment plan promptly, because the next step may be imaging rather than another blood sample. [Chronic Kidney Disease](/conditions/chronic-kidney-disease/) can affect which contrast-based tests or medicines are appropriate.

Call emergency services for sudden breathlessness, chest pain, coughing blood, fainting, severe weakness or a racing heartbeat. Seek urgent assessment for new one-sided leg swelling, warmth or pain, particularly after prolonged immobility. Do not wait for a D-dimer result if symptoms are severe or rapidly progressing.

Ask what clot is being considered, whether the blood test is appropriate, whether imaging is needed, when results will be available, and which symptoms require emergency action.