A D-dimer blood test measures fragments produced when the body breaks down a blood clot. It is mainly used when a clinician is assessing whether symptoms could be caused by a deep vein thrombosis or pulmonary embolism.
The most important point is that D-dimer is generally better at helping to rule out a blood clot than proving that one is present:
- A negative or low result can make a clot unlikely when your clinical risk is low or intermediate and the correct threshold has been used.
- A positive or raised result does not diagnose a clot. Imaging is usually needed if the clinician still suspects one.
- A normal result cannot always rule out a clot, particularly when clinical probability is high, symptoms have been present for some time or anticoagulant treatment has already begun.
D-dimer values and units vary between laboratories. Interpret the result using the reference range printed on your own report rather than comparing the number with an online example.
Call 999 if you have severe or unexplained breathlessness, chest pain, collapse, coughing up blood, a very fast heartbeat or other symptoms suggesting a pulmonary embolism. Do not wait for a private blood-test result or attempt to interpret one at home.
This guide explains D-dimer results in general terms. A result must be considered alongside your symptoms, examination, medical history and clinical probability of a clot.
What is a D-dimer blood test?
When the body forms a blood clot, strands of a protein called fibrin help hold it together. The body simultaneously activates processes that limit and eventually break down the clot.
D-dimer is one of the protein fragments released as cross-linked fibrin is broken down. An increased concentration suggests that clot formation and breakdown are occurring somewhere in the body.
This does not reveal:
- where the clotting activity is occurring;
- why it is happening;
- whether there is a dangerous clot;
- how large or old any clot might be.
Many illnesses and normal physiological states can activate clotting and inflammation. This makes D-dimer a sensitive but relatively non-specific test.
Why is the test requested?
D-dimer is most commonly used when a clinician is considering venous thromboembolism, or VTE. This includes:
- deep vein thrombosis (DVT), usually a blood clot in a deep vein of the leg;
- pulmonary embolism (PE), where clot material blocks an artery in the lungs.
Possible DVT symptoms include:
- swelling in one leg;
- calf or thigh pain and tenderness;
- warmth or redness;
- prominent surface veins;
- unexplained swelling of one arm in less common cases.
Possible PE symptoms include:
- sudden or unexplained breathlessness;
- chest pain, particularly pain that worsens when breathing in;
- coughing up blood;
- a fast heartbeat;
- dizziness, fainting or collapse;
- low oxygen levels;
- symptoms of a DVT alongside breathing difficulty.
D-dimer may also be measured when clinicians are investigating disseminated intravascular coagulation, a serious disturbance of clotting that can occur in severe illness. Its interpretation in that situation differs from its use for suspected DVT or PE.
The test is not designed as a general health screen for someone without relevant symptoms. An unexpected raised result can lead to anxiety and potentially unnecessary imaging without answering a useful clinical question.
What is a normal D-dimer result?
Many laboratories use a conventional cut-off equivalent to approximately 500 micrograms per litre fibrinogen-equivalent units, often shown as 500 µg/L FEU or 0.5 mg/L FEU.
However, this is not universal. Reports may use:
- µg/L FEU;
- ng/mL FEU;
- mg/L FEU;
- D-dimer units, abbreviated to DDU;
- a different assay-specific reference range.
| Example reporting format | Common conventional cut-off |
|---|---|
| µg/L FEU | Below 500 µg/L FEU |
| ng/mL FEU | Below 500 ng/mL FEU |
| mg/L FEU | Below 0.5 mg/L FEU |
These figures are examples only. A DDU result cannot necessarily be compared directly with an FEU threshold, and laboratories use different equipment and calibration methods.
Words such as “negative”, “within range” or “below the cut-off” can be more useful than the number in isolation. The clinician must still decide whether the test was appropriate for your level of risk.
Our general guide to understanding blood-test results explains reference ranges, units and flagged results.
What does a negative D-dimer mean?
A negative result means the D-dimer level was below the threshold used by the laboratory or clinical pathway.
In someone assessed as having a low or unlikely probability of DVT or PE, a negative high-sensitivity D-dimer can make an acute clot sufficiently unlikely that imaging is not required.
This is why the test is valuable: it can prevent people with a low clinical probability from undergoing unnecessary scans.
A negative result is less reassuring when:
- the clinical assessment suggests a high probability of a clot;
- symptoms have been present for a prolonged period;
- the suspected clot is small or confined to a limited area;
- anticoagulant medication was started before the sample was taken;
- the assay is less sensitive;
- the sample or result is technically unreliable.
A clinician may arrange imaging despite a normal D-dimer if symptoms and risk factors create substantial concern. NICE recommends using clinical-probability tools and defined diagnostic pathways rather than treating D-dimer as a standalone answer.
What does a raised D-dimer mean?
A raised result means more D-dimer was detected than expected under the threshold being applied. It shows increased fibrin formation and breakdown but does not establish the cause.
A high result can occur with DVT or PE, but it can also be caused by:
- recent surgery;
- physical injury or a fracture;
- pregnancy and the period after giving birth;
- increasing age;
- infection or sepsis;
- inflammatory conditions;
- cancer;
- liver disease;
- heart failure;
- recent bleeding;
- hospital admission or reduced mobility;
- a previous or resolving blood clot.
The result may therefore be raised in someone who does not have a new venous clot. This is especially common among older adults and people who are acutely unwell in hospital.
A mildly positive result is not automatically less important than a very high result. Although extremely elevated levels can be associated with substantial illness, the number alone cannot determine whether a clot is present or how dangerous it is.
What is an age-adjusted D-dimer threshold?
D-dimer levels tend to increase with age. Using the same cut-off for every adult can produce many positive results in older people who do not have a blood clot.
NICE advises clinicians to consider an age-adjusted threshold in people over 50. Where a particular FEU assay and pathway use the conventional 500 µg/L cut-off, a commonly used calculation is:
Age in years × 10 µg/L FEU
| Age | Example age-adjusted threshold |
|---|---|
| 50 or younger | 500 µg/L FEU |
| 60 | 600 µg/L FEU |
| 70 | 700 µg/L FEU |
| 80 | 800 µg/L FEU |
For example, a result of 620 µg/L FEU would be above a fixed threshold of 500 but below an example age-adjusted threshold of 700 for a 70-year-old.
Do not perform this calculation unless the laboratory assay and clinical pathway support it. Different units or DDU-based results require different interpretation.
Age adjustment is used to improve the test’s ability to rule out VTE without sending every older person for imaging. It does not mean that a raised D-dimer is normal for all purposes or that symptoms can be ignored.
See the current NICE recommendations on diagnosing venous thromboembolic disease.
How is D-dimer used with a clinical risk score?
Before interpreting D-dimer, the clinician considers symptoms, examination findings and risk factors. In UK pathways, this commonly includes a version of the Wells score for suspected DVT or PE.
Factors that may increase the probability of a clot include:
- previous DVT or PE;
- active cancer;
- recent surgery or prolonged immobility;
- one-sided leg swelling;
- tenderness along the deep veins;
- a fast heart rate;
- coughing up blood;
- clinical signs that make another diagnosis less likely.
The general approach is:
| Clinical situation | Likely next step |
|---|---|
| Clot considered unlikely | D-dimer may be used to help rule it out |
| Negative D-dimer in an appropriate low-risk patient | A new acute clot may be considered unlikely |
| Positive D-dimer | Imaging is usually considered |
| Clot considered likely | Proceed urgently towards imaging rather than relying on D-dimer |
The exact pathway differs between suspected DVT and suspected PE. It can also change according to local services, pregnancy, age and whether imaging is immediately available.
A risk score should be completed by a healthcare professional. It is not safe to calculate your own score and use it to decide against seeking care.
What happens after a positive D-dimer?
A positive test normally leads to further assessment rather than immediate confirmation of a clot.
For suspected DVT, the next investigation is usually a compression ultrasound scan of the leg veins. If the first scan is negative but the clinical situation and D-dimer remain concerning, a repeat scan may sometimes be arranged.
For suspected pulmonary embolism, imaging may include:
- CT pulmonary angiography, commonly called CTPA;
- a ventilation-perfusion or V/Q scan in selected patients;
- other investigations according to pregnancy, kidney function, chest X-ray findings and clinical circumstances.
Additional tests may include:
- a full blood count;
- kidney and liver blood tests;
- clotting tests;
- an ECG;
- oxygen-saturation measurement;
- a chest X-ray;
- troponin or other tests if the heart is under strain.
If imaging cannot be performed promptly and the clinical probability is sufficient, a clinician may begin interim anticoagulant treatment while the investigation is arranged. This decision must account for bleeding risk and other medical factors.
Do not begin aspirin or anticoagulant medicine yourself because of a raised D-dimer. Aspirin is not a substitute for appropriate treatment of suspected venous thromboembolism, and anticoagulants can cause serious bleeding.
Can D-dimer be normal when a clot is present?
Yes. No blood test detects every clot in every clinical situation.
A false-negative result is more likely when:
- the clot is small;
- symptoms began many days or weeks earlier;
- clot breakdown activity has already reduced;
- anticoagulants were taken before testing;
- the test has lower sensitivity;
- the value is close to the cut-off;
- the wrong threshold or units are applied.
This is why a normal result should not override a high-risk clinical picture. If symptoms worsen or new warning signs develop after a negative test, seek medical reassessment.
Similarly, having a negative result previously does not protect you from developing a clot later. A new episode of leg swelling, chest pain or breathlessness requires a new clinical assessment.
How are results interpreted during pregnancy?
D-dimer concentrations normally rise during pregnancy and may become particularly elevated in the third trimester and after delivery. A positive result is therefore common and less specific for a blood clot.
Pregnancy and the first weeks after birth also increase the risk of venous thromboembolism. Symptoms must not be dismissed simply because pregnancy itself can raise D-dimer.
Specialist pregnancy-adapted diagnostic pathways may use:
- the symptoms and examination findings;
- pregnancy-specific risk assessment;
- D-dimer in defined circumstances;
- ultrasound of the legs;
- V/Q scanning or CTPA where necessary.
Whether D-dimer is used varies between pathways and clinical settings. A standard non-pregnant reference range should not be interpreted in isolation.
Seek urgent maternity or emergency advice for one-sided leg swelling, unexplained chest pain, sudden breathlessness, coughing up blood, fainting or collapse during pregnancy or after giving birth.
What factors can affect the result?
D-dimer does not usually require fasting. Food and ordinary drinks are not important influences on the result, but the clinical timing and medical circumstances matter.
Tell the clinician about:
- when the symptoms started;
- recent surgery or injury;
- pregnancy or a recent birth;
- hospital admission or prolonged immobility;
- active or recent cancer treatment;
- infection or inflammatory illness;
- previous DVT or PE;
- anticoagulants or other medication;
- kidney, liver or heart disease.
A difficult blood draw or sample-handling problem can occasionally affect laboratory testing. The laboratory may request another sample if the specimen is unsuitable.
General influences on laboratory results are discussed in our guide to what can affect blood-test results.
Should you order a private D-dimer test?
D-dimer is not an ideal standalone home-screening test. Its value depends on a clinician assessing the probability of a clot before the blood is taken.
Potential problems with self-requested testing include:
- a positive result that is caused by something other than a clot;
- false reassurance from a negative result;
- use of the wrong reference threshold;
- delayed emergency assessment while waiting for the result;
- no immediate route to ultrasound or lung imaging;
- testing after symptoms have already been present for too long.
If you currently have symptoms suggesting DVT or PE, use an urgent clinical service rather than posting a sample to a laboratory. The clinician can assess the symptoms and organise immediate imaging or treatment when needed.
If a private test unexpectedly reports a raised result, contact the requesting clinician promptly. If you cannot reach them and you have potentially serious symptoms, contact NHS 111 or attend the urgent service recommended for your symptoms. Call 999 for severe breathing difficulty, collapse or other life-threatening signs.
Our guide to what to do after abnormal private blood-test results explains how to seek appropriate follow-up.
When should you seek urgent medical help?
Call 999 now if you have:
- sudden or severe difficulty breathing;
- chest pain with breathlessness;
- collapse, fainting or extreme weakness;
- coughing up blood with breathing symptoms;
- blue or grey lips or skin;
- a very fast heartbeat with chest pain or breathlessness;
- symptoms that feel immediately life-threatening.
Seek urgent same-day assessment if you develop:
- new swelling in one leg;
- unexplained calf or thigh pain with swelling or warmth;
- new one-sided arm swelling, particularly after a line or procedure;
- milder but unexplained breathlessness;
- chest pain when breathing;
- DVT symptoms after surgery, prolonged travel or immobility;
- possible clot symptoms during pregnancy or after birth.
Swelling in both ankles has many possible causes and is less typical of DVT, but it still deserves assessment if persistent or accompanied by breathlessness. See our guide to swollen legs and ankles.
Do not massage a painful, swollen leg in an attempt to disperse a suspected clot.
Frequently asked questions
Does a high D-dimer mean I have a blood clot?
No. It means clot formation and breakdown are increased, but many conditions can cause this. Imaging is normally needed to diagnose or exclude DVT or PE after a positive result.
Does a normal D-dimer rule out a clot?
It can help rule out an acute clot when the clinical probability is low or intermediate and a suitable high-sensitivity test and threshold are used. It may not be sufficient when clinical probability is high.
Is 500 a high D-dimer result?
Some laboratories use 500 µg/L FEU as a conventional cut-off, but the result depends on the units, assay, age-adjusted threshold and clinical pathway. Check the range printed on your report.
What is considered an extremely high D-dimer?
There is no single universal category because assays and units differ. A value many times above the laboratory cut-off needs clinical interpretation, but even a very high result does not identify the cause by itself.
Why is my D-dimer high without a clot?
Age, infection, inflammation, pregnancy, surgery, injury, cancer, liver disease and hospital admission can all raise the result without a new DVT or PE.
Can infection raise D-dimer?
Yes. Infection and severe inflammation can activate the clotting system and increase D-dimer. Clinicians still assess whether a clot could be present alongside the infection.
Can cancer cause a high D-dimer?
Yes, but a raised D-dimer is not a cancer test and does not mean that someone has cancer. It has many more common explanations and must be interpreted in context.
Does pregnancy raise D-dimer?
Yes. Levels commonly increase as pregnancy progresses and after delivery. Pregnancy-specific diagnostic pathways are therefore needed when a clot is suspected.
Do I need to fast for a D-dimer test?
No. Fasting is not usually necessary. Follow any separate instructions if other blood tests are being taken at the same time. Our guide to fasting before blood tests explains which tests may require it.
How quickly do D-dimer results come back?
Urgent hospital results may be available within hours or sooner, depending on the laboratory. Routine and private services may take longer. Someone with possible PE or DVT should not wait at home for a non-urgent result.
Can anticoagulants lower D-dimer?
They may reduce clotting activity and potentially lower D-dimer after treatment begins. Tell the clinician if you have already taken an anticoagulant.
Can aspirin lower D-dimer or treat a suspected clot?
Aspirin is not an appropriate substitute for assessment or anticoagulant treatment of suspected DVT or PE. Do not self-treat a raised result.
What scan follows a positive D-dimer?
A suspected DVT is usually investigated with a leg-vein ultrasound. Suspected PE is commonly investigated using CTPA or, in selected circumstances, a V/Q scan.
Should D-dimer be repeated?
Not routinely simply to see whether the number falls. Whether another test is useful depends on the clinical problem. NICE diagnostic pathways generally focus on appropriate imaging and assessment rather than serial self-testing.
Can D-dimer show where a clot is?
No. It is a blood marker and cannot locate a clot. Ultrasound, CTPA or another imaging test is needed to identify its location.
