D-dimer test explained

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Units: µg/l FEU
Updated: 2026-09-30; Every number with its source

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What D-dimers are and what the test shows

D-dimers are small protein fragments that form when the body breaks down (dissolves) a blood clot that has already formed — a process called fibrinolysis. In the blood of a healthy person there is very little D-dimer or none at all, but when a clot appears in the body — for example deep vein thrombosis (DVT) or pulmonary embolism — the amount in the blood rises.

According to MedlinePlus, a D-dimer test does not establish where exactly the clot is, but it helps the doctor decide whether further, more precise tests are needed (for example an ultrasound of the veins or a CT scan of the chest). The test is also used to monitor the course of treatment of disseminated intravascular coagulation (DIC), a severe condition of the clotting system. A D-dimer test is sensitive but not specific: a raised result may mean a clot, but it can also arise from many other causes unrelated to dangerous thrombosis, so a single result does not by itself confirm a diagnosis.

How the result is read

A normal (negative) D-dimer result makes deep vein thrombosis or pulmonary embolism unlikely only when the doctor's clinical assessment (for example, the Wells score) also shows a clot to be unlikely (NICE NG158), while a positive (raised) result usually means that further tests are needed to confirm or rule out a clot. Lab Tests Online gives a commonly used cut-off of 0.50 mg/l FEU (500 µg/l FEU): below it the result is read as negative, above it as positive. Cut-offs and units still differ between laboratories and methods, so you will find the exact reference limit on your own result sheet.

Result	What it usually means	What usually follows
Commonly used cut-off	Below 0.50 mg/l FEU (500 µg/l FEU) is read as negative, above it as positive (Lab Tests Online, citing the American Board of Internal Medicine)	Cut-offs and units differ between laboratories — check the limit on your own report
Negative (low)	A clot is unlikely only if the doctor's clinical assessment also shows a low probability (NICE NG158); other clotting disorders are not ruled out	The final judgement on whether additional tests are needed is made by a doctor, taking the symptoms and other risk factors into account
Positive (raised)	A clot may have formed, but the result is not specific	Further, more precise tests to confirm or rule out a clot

When the test is done

When there is swelling of a leg or an arm, especially on one side only — when deep vein thrombosis is suspected (in that case see a doctor without delay)
When chest pain or sudden shortness of breath appears — when pulmonary embolism is suspected (in that case do not wait for a test — call 112)
When there are signs of unusual bleeding, for example bleeding from the gums
To monitor the course of treatment of disseminated intravascular coagulation (DIC)
When a doctor is assessing overall clotting together with other coagulation markers

How to prepare

According to MedlinePlus, no special preparation is needed for a D-dimer test
It is worth mentioning recent surgery, an injury or pregnancy to your doctor — these factors can affect the result

A raised (positive) result

Raised D-dimers may indicate that a blood clot could have formed in the body, but the result is not specific — it can be affected by age, pregnancy, recent surgery and other conditions. According to MedlinePlus, a positive (raised) result usually means that further tests are needed to confirm or rule out a clot.

Causes of a raised result

Blood clotting disorders — deep vein thrombosis, pulmonary embolism, disseminated intravascular coagulation (DIC), stroke
Older age
Pregnancy
Recent surgery or injury
Heart disease
Rheumatoid arthritis
Long-term restricted movement (immobilisation)
Infection, a heart attack, some cancers or liver disease (according to Lab Tests Online)

What to discuss with a doctor

Imaging to confirm a clot — for example an ultrasound of the veins or a CT scan of the chest
Other coagulation markers (for example fibrinogen, prothrombin time, APTT) for the overall picture
Whether there has been recent surgery, an injury or a pregnancy that could explain the result
When to repeat the test, if DIC or another clotting condition is being monitored

How the D-dimer test differs from other clotting tests

D-dimers show whether the body has recently been breaking down a blood clot, whereas fibrinogen and prothrombin time (INR) assess the function of the clotting process itself. These markers are often assessed together to give an overall picture of the clotting system.

A single D-dimer result does not by itself confirm a diagnosis. A low or normal result makes a clot unlikely only when the doctor's clinical assessment also shows a low probability (NICE NG158), and the final judgement on whether additional tests are needed is made by a doctor, taking the symptoms and other risk factors into account.

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Frequently asked questions

›What is the normal D-dimer range?

According to Lab Tests Online, a commonly used cut-off is 0.50 mg/l FEU (500 µg/l FEU): below it the result is read as negative, above it as positive. A negative result makes deep vein thrombosis or pulmonary embolism unlikely only when the doctor's clinical assessment (for example, the Wells score) also shows a clot to be unlikely (NICE NG158); for people over 50 an age-adjusted cut-off may be used. It does not rule out other clotting disorders, such as an inherited tendency to thrombosis or bleeding disorders. Cut-offs and units depend on the method the laboratory uses, so you will find the exact reference limit on your own result sheet.

›What does a raised D-dimer mean?

Raised (positive) D-dimers may indicate that a blood clot has formed in the body, for example deep vein thrombosis or pulmonary embolism, but the result can also be affected by other causes — age, pregnancy, recent surgery. That is why a positive result usually means further, more precise tests.

›Do I need to prepare before a D-dimer test?

No, according to MedlinePlus no special preparation is needed for a D-dimer test — no fasting and no other restrictions.

›Why does a doctor order a D-dimer test?

A D-dimer test is most often ordered when there are symptoms of a possible blood clot — swelling of a leg or an arm, chest pain, sudden shortness of breath or unusual bleeding. It is also used to monitor the course of treatment of disseminated intravascular coagulation. If chest pain or shortness of breath comes on suddenly, do not wait for a test — call 112.

›Does a low D-dimer result completely rule out a blood clot?

No. A low or normal result makes a clot unlikely only when the doctor's clinical assessment (for example, the Wells score) also shows a low probability (NICE NG158); it does not rule out other clotting disorders, and the final judgement on whether additional tests are needed is made by a doctor, taking the symptoms and other risk factors into account.

›How does a D-dimer test differ from fibrinogen or prothrombin time tests?

Sources

Related markers

This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.

This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.