D-Dimer (D-D)

D-dimer is a small protein fragment produced when a blood clot breaks down in the body. Under normal circumstances very little is present in the blood, so a raised level suggests that clot formation and breakdown may be taking place. This makes D-dimer a useful test in the investigation of conditions involving abnormal blood clotting.

What it can help assess

• Investigating suspected blood clots: D-dimer is used to help assess conditions such as deep vein thrombosis (DVT), a clot usually in the leg, and pulmonary embolism (PE), a clot in the lungs
• Ruling out a clot: its main strength is that a normal result makes a significant clot unlikely, which is clinically very useful
• Supporting the assessment of clotting disorders, alongside other tests

An important note on interpretation

D-dimer is a sensitive but non-specific test. This means a normal result is reassuring and helps rule out a clot, but a raised result does not confirm one, as levels can rise for many other reasons. These include infection, inflammation, recent surgery or injury, pregnancy, and increasing age. For this reason, a raised D-dimer usually means further investigation such as a scan is needed, rather than giving a diagnosis on its own, and it is always interpreted alongside your symptoms and clinical assessment.

Why it matters

D-dimer is a valuable test for helping to assess whether a harmful blood clot may be present. Because a suspected clot can be serious, this test is most meaningful when interpreted by a clinician as part of a full assessment. Any raised or abnormal result should be discussed promptly with a doctor, who can arrange any further investigation needed.