Finding the right tests…
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Emergency services available 24 x 7
Finding the right tests…
Serology
D-dimer measures a small protein fragment left behind when your body dissolves a blood clot. Because the fragment only appears once a clot has formed and then started breaking down, doctors use this test mainly to help rule out a clot in the deep veins of the leg or a clot that has travelled to the lungs, especially in someone whose symptoms and risk factors already suggest a low chance of either. A normal result in that setting makes a clot unlikely enough that further scans can often be skipped. An elevated result does not confirm a clot - it means an ultrasound, CT scan, or similar imaging test is needed next.
The test reports a level of D-dimer in your plasma, usually compared against a single cutoff rather than a range - below it is read as normal, above it as elevated. A normal result carries real weight in a patient judged unlikely to have a clot before testing, since D-dimer is very often normal when no clot is present. An elevated result is common and often unrelated to clotting - recent surgery, pregnancy, infection, cancer, and simply being older can all raise it, so the number alone cannot say whether a clot is the cause.
D-dimer is built to catch clot breakdown, but many things besides a dangerous clot can raise it. That makes an elevated result common and not very specific, while a normal result in someone with a low pre-test likelihood of clotting is genuinely reassuring - one reason emergency departments lean on this test to avoid unnecessary CT scans in low-risk patients.
- Recent surgery or a major injury - Pregnancy, particularly later trimesters - Active cancer - Infection or inflammation - Older age, even in someone healthy
If you are already taking a blood thinner when this test is drawn, a clot can be actively dissolving under treatment and the result can still come back normal, which is why doctors interpret a normal D-dimer differently in someone already on anticoagulants versus someone who is not. Your doctor combines the result with a clinical probability score based on your symptoms and history, rather than reading the number alone.
In someone judged low or moderate risk before testing, a D-dimer under roughly 500 ng/mL is generally considered enough to rule out a clot in the legs or lungs without further imaging. Ranges and cutoffs vary by lab and by the specific assay used, so always check what your own report states.
Recent surgery, pregnancy, active cancer, infection, and older age can all raise D-dimer without a dangerous clot being present. This is why an elevated result on its own leads to further imaging rather than an immediate diagnosis.
Yes, though your doctor interprets the result differently if you're already on a blood thinner, since treatment can lower D-dimer even when a clot is present. Tell your doctor about any blood thinners you're taking before the test.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
There is no fixed fasting requirement for this test. Ask your doctor or the centre when you book, particularly if it's being drawn alongside other blood work.
Pregnancy naturally increases clotting activity as the body prepares for delivery, which raises D-dimer even without any clotting problem. Doctors account for this when interpreting results in pregnant patients.
An elevated result usually leads to an imaging test, such as a leg ultrasound or a CT scan of the chest, to check directly for a clot. The D-dimer result itself does not confirm or rule out a clot on its own.