D-dimer is a fibrin degradation product — a fragment of protein released when a blood clot dissolves. The test is primarily used in emergency and hospital settings to rule out blood clots (deep vein thrombosis and pulmonary embolism) in patients with compatible symptoms.
What a normal result means
A D-dimer below the lab's cut-off (typically 0.5 mg/L or 500 ng/mL, though this varies) effectively rules out a significant clot in someone with a low pre-test probability. This is valuable because a negative result avoids the need for more invasive imaging. The D-dimer test has a high negative predictive value — if it's normal and clinical suspicion is low, a clot is very unlikely.
Why a raised D-dimer doesn't mean a clot
D-dimer rises whenever clotting is occurring anywhere in the body — not just in veins. Infection, inflammation, pregnancy, recent surgery, trauma, cancer and even age can all elevate D-dimer. The test is highly sensitive but not specific: it catches almost all clots but also flags many non-clot conditions.
This is why a raised D-dimer is the beginning of an investigation, not a diagnosis. Your doctor will use a clinical probability score (such as the Wells score) alongside the D-dimer result to decide whether imaging — typically a CT pulmonary angiogram for PE or a leg ultrasound for DVT — is needed.
- Infection or sepsis — inflammatory clotting cascade is activated
- Pregnancy — D-dimer rises naturally and may be elevated throughout
- Recent surgery or trauma — clotting is part of the healing response
- Cancer — especially lung, pancreatic and colorectal cancers activate coagulation
- Age — D-dimer reference ranges are sometimes age-adjusted (age × 10 µg/L in adults over 50)