D-dimer: what it is, what it tells you and when it’s tested

If you have a swollen, warm, painful leg, sudden breathlessness, chest pain or you are coughing up blood, do not rely on a test: call 112 or go to the emergency department. D-dimer is a small piece of dissolved clot that circulates in your blood. It is mainly measured to rule out a blood clot, and your doctor reads it together with an examination.

When it’s tested

Doctors order it when they suspect a clot in a leg vein (deep vein thrombosis) or in the lungs (pulmonary embolism). They first judge how likely it is, then decide whether a D-dimer is useful.

It is most useful when it is negative: if the risk is low and the D-dimer is normal, a clot is very unlikely and often no other tests are needed.

It is not a test for people who feel well. It rises for many harmless reasons, and a high value without symptoms often leads to unnecessary tests and worry. That is why it is not in the Rosa panel.

What it tells you, and what it does not

If it is normal, and your doctor thinks the risk is low, a clot is very unlikely.

If it is high, it does not mean you have a clot. It also rises with age, in pregnancy, after surgery, and with an infection or inflammation. Your doctor decides whether more tests are needed, such as a leg ultrasound or a CT scan.

If your doctor already thinks a clot is likely, they often skip it and go straight to imaging.

That is why it is read together with other tests:

Normal D-dimer levels

The most common cut-off is 500 ng/mL FEU, which is the same as 0.5 mg/L FEU. Below it, the D-dimer counts as negative.

Watch the unit. Some labs use a different unit (DDU), with values about half as large. The limit printed on your own report is what counts.

D-dimer rises on its own with age. From 50, many doctors use your age multiplied by 10 as the cut-off: at 70, for example, 700 ng/mL FEU. In pregnancy it rises on its own, so a high value is common and says little.

If your D-dimer is high or low

What can raise it without a clot

Many things raise it. That is why it is not enough on its own:

When to worry

How you feel counts more than the number:

Frequently asked questions

What are normal D-dimer levels? The most common cut-off is 500 ng/mL FEU, or 0.5 mg/L. From 50, many doctors use your age times 10. Always check the unit and the limit on your own report.

Does a negative D-dimer rule out a clot? If your doctor thinks the risk is low, almost always, yes. If the risk is high, your doctor goes on to imaging anyway.

Does D-dimer rise after COVID? During COVID and other infections it can rise, sometimes a lot. After you recover it usually returns to normal. If you have symptoms of a clot, go to the emergency department all the same.

Do I need to fast for a D-dimer test? No. It is a normal blood test, often done in the emergency department.

Does Rosa test D-dimer? No. It is a test for when a clot is suspected, not for prevention. Rosa measures platelets and high-sensitivity CRP, along with the rest of the panel.

Sources