What is D-dimer?
When a blood clot (thrombus) forms, a protein called fibrin creates a mesh to hold it together. When the body naturally dissolves a clot through fibrinolysis, fibrin cross-links are broken down into D-dimer fragments, which enter the bloodstream. The D-dimer test detects these fragments. It is highly sensitive but not specific — meaning a normal D-dimer strongly suggests no significant clot, but a high D-dimer does not confirm a clot (many other conditions raise it). The test is most useful in ruling OUT clotting disorders in low-to-moderate risk patients.
D-dimer normal range
| Result | Value (mg/L FEU) | Interpretation |
|---|---|---|
| Normal | < 0.50 | Clot unlikely in low-risk patients |
| Elevated | 0.50 – 1.0 | Borderline; assess clinical probability |
| High | > 1.0 | Further imaging needed |
| Age-adjusted cut-off (if >50 yrs) | Age x 0.01 mg/L | Higher threshold reduces unnecessary imaging |
Causes of high D-dimer
| Condition | D-dimer level |
|---|---|
| DVT (deep vein thrombosis) | Elevated |
| Pulmonary embolism (PE) | Elevated |
| Atrial fibrillation | Mildly elevated |
| COVID-19 | Often significantly elevated |
| Sepsis and severe infection | Elevated |
| DIC (disseminated intravascular coagulation) | Very high |
| Cancer (especially metastatic) | Elevated |
| Pregnancy (especially 3rd trimester) | Naturally elevated |
| Post-surgery / trauma | Elevated for weeks |
| Stroke | Elevated |
| Liver disease | Elevated (reduced clearance) |
| Inflammation (CRP elevated) | Mildly elevated |
A normal D-dimer is very reassuring
In patients with a low pre-test probability of DVT or PE (assessed using the Wells score), a normal D-dimer (<0.5 mg/L) reliably rules out a clinically significant blood clot. This avoids unnecessary CT or ultrasound scans. A high D-dimer in a high-risk patient requires imaging — a CT pulmonary angiogram (CTPA) for suspected PE or a Doppler ultrasound for suspected DVT.
Questions to ask your doctor
- What is my pre-test probability (Wells score)?
- Do I need a CT chest (CTPA) or leg ultrasound?
- Could my D-dimer be elevated for a reason other than a clot?
- If I have a clot, how long do I need blood thinners?