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.

• D-Dimer

No special preparation required.

For blood tests, we recommend drinking plenty of water beforehand to ensure smooth sample collection. If you are taking any regular medication, continue to do so unless advised otherwise by your doctor.