What Does D-Dimer Mean on a Blood Test?

What it means

When a blood clot forms and the body starts to dissolve it, small protein fragments called D-dimers are released into the blood. The test measures how much of these fragments is present. It is often used when a doctor is checking for a clot in a leg vein (deep vein thrombosis) or in the lungs (pulmonary embolism), which can need urgent medical help. Sudden breathlessness, chest pain or a painful, swollen leg should be checked urgently, whatever an earlier test showed.

D-dimer is mainly a "rule-out" test. When a doctor already thinks a clot is fairly unlikely, a negative D-dimer helps them feel confident there is no clot, so a scan may not be needed. A positive or raised D-dimer does not show where a clot is, or even that there is one. It means more checks, usually a scan, are needed to find out.

D-dimer is different from clotting tests such as INR or APTT. Those measure how quickly blood clots in a tube, while D-dimer looks for evidence that clots have been forming and breaking down in the body.

Where you may see it

• Emergency department blood results when a clot in the leg or lung is being considered
• Hospital blood tests during serious infections, including COVID-19
• Pregnancy and after-surgery blood tests
• Reports that note the result is "age-adjusted" for people over a certain age