Normal Range for D-dimer
The normal range for D-dimer is typically less than 0.5 μg/mL (or 500 ng/mL), with values above this threshold considered elevated and potentially indicative of active coagulation and fibrinolysis. 1, 2
Understanding D-dimer Measurement
- D-dimer is a fibrin degradation product that results from the breakdown of blood clots through the fibrinolytic system 2
- D-dimer has a relatively long half-life in circulation (approximately 16 hours), making it detectable even after the initial thrombotic event 2
- D-dimer testing is commonly used to rule out venous thromboembolism in patients with low clinical probability 1
Important Considerations for D-dimer Interpretation
Units and Reporting Variations
- D-dimer can be reported in two different unit types 1:
- Fibrinogen Equivalent Units (FEU)
- D-dimer Units (DDU)
- FEU is approximately two-fold higher than DDU, which can cause significant confusion if the unit type is not clearly specified 1
- There is tremendous variability in the magnitude of units reported (ng/mL, μg/mL, mg/L, μg/L), further complicating interpretation 1
Age-Adjusted Cutoffs
- For patients over 50 years of age, an age-adjusted cutoff (age × 10 ng/mL) is recommended to improve specificity while maintaining high sensitivity 2
- D-dimer specificity decreases steadily with age, reaching as low as 10% in patients over 80 years old 2
Special Populations
- Pregnancy: D-dimer levels increase progressively during pregnancy 2:
- First trimester: 0.11-0.40 μg/mL
- Second trimester: 0.14-0.75 μg/mL
- Third trimester: 0.16-1.3 μg/mL (up to 2 μg/mL may still be normal)
Clinical Significance of Elevated D-dimer
- Mildly elevated D-dimer (>0.5 μg/mL) has high sensitivity (95-99%) but poor specificity (40%) for venous thromboembolism 3, 4
- Moderately elevated D-dimer (>3.6 μg/mL) has increased specificity (78.3%) for deep vein thrombosis while maintaining good sensitivity (93.8%) 5
- Extremely elevated D-dimer (>5000 μg/mL or >10x normal) is highly specific for serious conditions 6:
- 89% of patients with extremely elevated D-dimer have venous thromboembolism, sepsis, and/or cancer
- Common diagnoses include pulmonary embolism (32%), cancer (29%), sepsis (24%), and deep vein thrombosis (13%)
Pitfalls and Caveats
- D-dimer cutoff values are not transferable between different assay methods or between institutions 1
- D-dimer levels can be elevated in numerous non-thrombotic conditions 2, 4:
- Advanced age
- Pregnancy
- Recent surgery or trauma
- Inflammatory states
- Infections
- Malignancy
- Hospitalized patients (only 22% of hospitalized patients without VTE have normal D-dimer levels) 4
- Point-of-care D-dimer assays have lower sensitivity compared to laboratory-based tests 2
- Different laboratories may use different reporting units and reference ranges, making it essential to know the specific reference range for the laboratory performing the test 1