Initial Laboratory Tests for Diagnosing Deep Vein Thrombosis (DVT)
The primary initial laboratory test for diagnosing DVT is a highly sensitive D-dimer assay, which should be used in conjunction with clinical probability assessment to guide further diagnostic testing. 1
Diagnostic Algorithm Based on Clinical Probability
Step 1: Clinical Probability Assessment
First, assess the pretest probability of DVT using a validated clinical decision rule (such as Wells score) that stratifies patients into low, moderate, or high probability categories.
Step 2: Initial Testing Based on Pretest Probability
Low Pretest Probability:
- Recommended initial test: Highly sensitive D-dimer (Grade 2B) or moderately sensitive D-dimer (Grade 2C) 1
- If D-dimer is negative: No further testing needed (Grade 1B)
- If D-dimer is positive: Proceed to compression ultrasonography (CUS) of proximal veins
Moderate Pretest Probability:
- Recommended initial test: Highly sensitive D-dimer (Grade 2C) 1
- If D-dimer is negative: No further testing needed (Grade 1B)
- If D-dimer is positive: Proceed to CUS
High Pretest Probability:
- Recommended initial test: Proximal CUS or whole-leg ultrasound (Grade 1B) 1
- D-dimer testing is not recommended as the initial test in high probability patients
Advantages of D-dimer Testing
- A negative highly sensitive D-dimer test effectively rules out DVT in patients with low or moderate pretest probability
- Can reduce the need for ultrasound testing by approximately 30% 2
- Provides excellent negative predictive value (96.8% for highly sensitive assays) 3
Important Considerations
Test selection: Choose highly sensitive D-dimer assays over moderately sensitive ones
False positives: D-dimer levels may be elevated in conditions unrelated to DVT:
- Advanced age
- Pregnancy
- Recent surgery
- Malignancy
- Inflammation
- Infection
Limitations: The SimpliRED D-dimer assay has insufficient sensitivity (66%) to be used as a reliable exclusion test 3
Ultrasound limitations: When ultrasound is impractical (e.g., leg casting, excessive subcutaneous tissue) or nondiagnostic, CT venography or MR venography should be considered 4
Special Populations
Pregnant Patients
- Initial evaluation with proximal CUS is recommended over D-dimer testing (Grade 1B) 1
- D-dimer levels are often elevated in normal pregnancy, reducing specificity
Suspected Recurrent DVT
- D-dimer testing can be useful when combined with ultrasound findings
- For abnormal but nondiagnostic ultrasound results, venography or serial proximal CUS with D-dimer testing is recommended 1
Suspected Iliocaval DVT
- When whole-leg swelling is present with normal compression ultrasound, CT or MR venography is strongly recommended 4
Conclusion
The combination of clinical probability assessment and D-dimer testing provides a safe and effective initial approach to DVT diagnosis, potentially avoiding unnecessary imaging in patients with low to moderate pretest probability and negative D-dimer results.