Unit Converter
D-Dimer
(Fibrin Degradation Product – Marker for Thrombosis, DIC & VTE Exclusion)
Synonyms
- D-dimer
- Fibrin D-dimer
- Cross-linked fibrin degradation product
- FDP D-dimer
- Fibrin fragment D
- Fibrin split product
Units of Measurement
Non-FEU (standard mass units)
- μg/mL
- ng/mL
- mg/L
- μg/L
FEU Units (Fibrinogen Equivalent Units)
(Most widely used in modern assays)
- μg FEU/mL
- ng FEU/mL
- mg FEU/L
- μg FEU/L
1 FEU = amount of D-dimer derived from 1 unit of fibrinogen
1 μg/mL FEU = 1000 ng/mL FEU = 1 mg/L FEU
Description
D-dimer is a specific breakdown product of cross-linked fibrin formed during fibrinolysis. It reflects:
- Activation of coagulation (clot formation)
- Activation of plasmin/fibrinolysis (clot breakdown)
D-dimer is used primarily to exclude venous thromboembolism (VTE) when pre-test probability is low or moderate.
Primary clinical uses:
- Exclusion of deep vein thrombosis (DVT)
- Exclusion of pulmonary embolism (PE)
- Diagnosis/monitoring of disseminated intravascular coagulation (DIC)
- Coagulopathy in COVID-19, sepsis, trauma
- Post-surgical or post-thrombotic monitoring
Physiological Role
D-dimer is formed when:
- Fibrinogen → fibrin (via thrombin)
- Factor XIII cross-links fibrin
- Plasmin breaks it down → releases D-dimer fragments
Thus D-dimer is a marker of active clot formation AND breakdown.
Clinical Significance
High D-Dimer
(Most important – indicates increased clot turnover)
1. Venous Thromboembolism
- Deep vein thrombosis (DVT)
- Pulmonary embolism (PE)
- Used with Wells score → if low probability + low D-dimer = rule out VTE
2. Disseminated Intravascular Coagulation (DIC)
Extremely elevated values; used with:
- Platelets
- PT, aPTT
- Fibrinogen
3. Trauma / Surgery / Burns
Increased due to tissue injury.
4. Pregnancy
Physiological rise (up to 4–5×).
5. Malignancy
Especially advanced cancers.
6. Infections
Sepsis, pneumonia, COVID-19.
7. Liver disease
Due to impaired clearance.
8. Age
Natural increase with aging.
Low/Normal D-Dimer
Helps exclude VTE when paired with a low pretest probability.
Reference Intervals & Clinical Cutoffs
(ISTH + ACEP + ACCP + Mayo + ARUP)
Normal Range
- < 500 ng/mL FEU
(or <0.5 μg/mL FEU or <0.5 mg/L FEU)
Age-Adjusted D-Dimer Cutoff (recommended for >50 years)
Cutoff (ng/mL FEU)=Age×10\text{Cutoff (ng/mL FEU)} = \text{Age} \times 10Cutoff (ng/mL FEU)=Age×10
Example: Age 70 → cutoff = 700 ng/mL FEU
DIC
Levels often >4000–10,000 ng/mL FEU
Mass Units Equivalence
| Unit | Equivalent to 0.5 mg/L FEU |
| 0.5 mg/L FEU | = 500 ng/mL FEU |
| = 0.5 μg/mL FEU | = 500 μg/L FEU |
Conversions (FEU standard)
1 μg/mL FEU=1000 ng/mL FEU1\ \text{μg/mL FEU} = 1000\ \text{ng/mL FEU}1 μg/mL FEU=1000 ng/mL FEU 1 mg/L FEU=1000 μg/L FEU1\ \text{mg/L FEU} = 1000\ \text{μg/L FEU}1 mg/L FEU=1000 μg/L FEU
Non-FEU (D-dimer mass units)
Some assays report D-dimer directly (not FEU):
1 μg/mL≠1 μg/mL FEU1\ \text{μg/mL} \neq 1\ \text{μg/mL FEU}1 μg/mL=1 μg/mL FEU
Conversion typically:
1 μg/mL D-dimer≈2 μg/mL FEU1\ \text{μg/mL D-dimer} ≈ 2\ \text{μg/mL FEU}1 μg/mL D-dimer≈2 μg/mL FEU
(Manufacturer-specific; always check lab method.)
Diagnostic Uses
1. Excluding DVT/PE
- Must combine with Wells score
- Low Wells + D-dimer <500 ng/mL FEU = VTE ruled out
2. Diagnosing/Monitoring DIC
Used in DIC scoring systems with:
- Platelets
- PT
- Fibrinogen
3. COVID-19 Coagulopathy
Higher levels correlate with:
- ARDS
- ICU need
- Mortality
4. Post-operative Monitoring
Detects excessive clot formation.
5. Pregnancy Complications
Elevated in:
- Pre-eclampsia
- Placental abruption
- DIC
6. Cancer-Associated Thrombosis
Analytical Notes
- Plasma (citrate) is preferred
- Immunoturbidimetric and ELISA assays used
- Lipemia/hemolysis interfere minimally
- Always specify FEU vs non-FEU units
- D-dimer is highly sensitive but not specific
Clinical Pearls
- A negative D-dimer is more powerful than a positive one.
- D-dimer is almost always high in hospitalized patients-interpret with context.
- Use age-adjusted cutoffs in older adults to avoid unnecessary imaging.
- Very high D-dimer does not specify the cause - think DIC, PE/DVT, cancer, sepsis.
- After 3–6 days of VTE symptoms, D-dimer sensitivity decreases.
Interesting Fact
The D-dimer test became globally recognized during the COVID-19 pandemic, where high levels predicted severity and mortality - leading it to become one of the most ordered lab tests worldwide.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Coagulation Markers.
- ISTH Guidelines - D-dimer Use in VTE & DIC.
- ACCP Antithrombotic Guidelines.
- ACEP Clinical Policy - D-dimer in PE/DVT.
- Mayo Clinic Laboratories - D-dimer.
- ARUP Consult - D-dimer.
- NIH / MedlinePlus - D-dimer Test.