Blood test
D-dimer Test
A D-dimer test measures a substance released when a blood clot breaks down. It is mainly used to help rule out clots such as DVT and pulmonary embolism.
Quick answer
D-dimer Test: what it is and what the results mean
D-dimer is a fragment produced when a blood clot dissolves. A D-dimer blood test measures its level and is used chiefly to help decide whether a clot is likely.
- Why it is done: It is used mainly when a deep vein thrombosis (DVT) or pulmonary embolism (a clot in the lung) is suspected, to help rule these out when the level is low, alongside a clinical assessment.
- Understanding results: A low D-dimer, in someone assessed as low risk, makes a significant clot unlikely and can avoid further scans.
What it is
D-dimer is a fragment produced when a blood clot dissolves. A D-dimer blood test measures its level and is used chiefly to help decide whether a clot is likely.
Why it is done
It is used mainly when a deep vein thrombosis (DVT) or pulmonary embolism (a clot in the lung) is suspected, to help rule these out when the level is low, alongside a clinical assessment.
What to expect
A standard blood sample is taken from the arm, usually in an emergency or acute setting, with results available quickly.
Understanding the results
A low D-dimer, in someone assessed as low risk, makes a significant clot unlikely and can avoid further scans. A raised D-dimer is non-specific — it can rise with infection, injury, surgery or pregnancy — so it usually prompts imaging rather than confirming a clot.
Good to know
Risks and limitations
The blood test is very safe. Its value is mainly in ruling out clots; because many things raise D-dimer, a high result is not a diagnosis and needs further tests such as an ultrasound or CT scan.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
D-dimer Test: frequently asked questions
What does a positive D-dimer mean?
A raised D-dimer suggests a clot may be present but is not specific — infection, injury, surgery and pregnancy also raise it. It usually leads to a scan to check for a clot rather than confirming one.
Can a normal D-dimer rule out a clot?
In someone assessed as low risk, a normal D-dimer makes a significant clot unlikely and can avoid further scans. It is always used alongside a clinical risk assessment, not on its own.
Related tests
Other blood test investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Relevant royal college / professional body
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