Negative D-dimer Does Not Rule Out DVT in All Cases and Prophylactic Anticoagulation Should Be Considered
A negative D-dimer alone is not sufficient to rule out deep vein thrombosis (DVT) in a patient with a swollen leg, and prophylactic enoxaparin (Clexane) should be considered until the scan results are available, especially if the patient has a high pretest probability. 1
Diagnostic Approach to Suspected DVT
Risk Stratification
- The American College of Chest Physicians recommends using the Wells score to stratify patients into probability categories 1:
- Score ≥2: High pretest probability
- Score <2: Low pretest probability
- Wells score factors include:
- Active cancer (1 point)
- Paralysis/recent immobilization (1 point)
- Bedridden ≥3 days or major surgery within 12 weeks (1 point)
- Localized tenderness along deep veins (1 point)
- Entire leg swollen (1 point)
- Calf swelling ≥3 cm larger than asymptomatic side (1 point)
- Pitting edema confined to symptomatic leg (1 point)
- Collateral superficial veins (1 point)
- Previous DVT (1 point)
- Alternative diagnosis at least as likely (-2 points)
Role of D-dimer Testing
- For patients with low pretest probability, a negative D-dimer can safely rule out DVT 1
- For patients with high pretest probability, D-dimer should not be used as a stand-alone test to rule out DVT 2, 1
- The American College of Chest Physicians specifically states that "moderately or highly sensitive D-dimer assays should not be used as stand-alone tests to rule out DVT" in high pretest probability patients 2
Imaging Recommendations
- For patients with high pretest probability, proximal compression ultrasound (CUS) or whole-leg ultrasound is recommended as the initial test 2, 1
- If initial proximal CUS is negative but clinical suspicion remains high, additional testing with repeat CUS in 1 week, whole-leg US, or D-dimer testing is recommended 2
Management Until Scan Results
Prophylactic Anticoagulation
- Given the swollen leg and pending scan, prophylactic enoxaparin (Clexane) would be reasonable until definitive diagnosis, especially if the patient has high pretest probability 3
- The standard prophylactic dose of enoxaparin is 40 mg subcutaneously once daily 3
- This approach balances the risk of thromboembolism against the risk of bleeding while awaiting definitive diagnosis
Important Considerations
- Asymmetric calf swelling of >2 cm is present in 64% of patients with proximal DVT compared to only 16% without DVT 4
- If the patient has extensive unexplained swelling, iliac vein imaging should be considered to exclude isolated iliac DVT 1
Common Pitfalls to Avoid
- Overreliance on D-dimer: A negative D-dimer in a patient with high pretest probability does not reliably exclude DVT 2, 1
- Failure to assess pretest probability: Using the Wells score is essential for proper interpretation of test results 1
- Delayed diagnosis: Withholding prophylactic anticoagulation in high-risk patients while awaiting imaging could lead to clot propagation or embolization 3
- Ignoring clinical assessment: The combination of clinical probability and D-dimer is more reliable than either alone 5
Remember that while a negative comprehensive ultrasound has a high negative predictive value (99.6%) 6, the patient in question has not yet had the scan, and the D-dimer result must be interpreted in the context of clinical probability.