Duplex Ultrasound is the Next Step
The best next step is C: duplex ultrasound of the left lower extremity. This patient has a high pretest probability for deep venous thrombosis (DVT) based on recent major surgery (hip replacement 6 weeks ago), entire leg swelling, and pitting edema, which warrants immediate diagnostic imaging rather than empiric anticoagulation or D-dimer testing 1.
Risk Stratification Using Wells Score
This patient's Wells score is at least 3 points, indicating high pretest probability for DVT 1:
- Recently bedridden/major surgery within 12 weeks: +1 point 1
- Entire leg swollen: +1 point 1
- Pitting edema confined to symptomatic leg: +1 point 1
The large hematoma at the surgical site does not constitute an "alternative diagnosis at least as likely as DVT" because post-surgical hematomas and DVT commonly coexist, and the entire leg swelling suggests a more proximal venous process 1.
Why Duplex Ultrasound Over Other Options
Complete duplex ultrasound (CDUS) is the preferred initial test for patients with high pretest probability of DVT 1. The American College of Chest Physicians explicitly recommends proximal compression ultrasound (CUS) or whole-leg ultrasound over no testing (Grade 1B) in patients with high pretest probability 1.
Why Not D-dimer (Option D)?
D-dimer should NOT be used as a stand-alone test in patients with high pretest probability (Grade 1B) 1. Even a negative D-dimer cannot safely exclude DVT when clinical suspicion is high, and this patient's recent major orthopedic surgery makes D-dimer particularly unreliable as it remains elevated for weeks post-operatively 1.
Why Not Heparin Drip (Option B)?
Starting empiric anticoagulation without diagnostic confirmation is inappropriate when ultrasound is readily available 1. The American College of Chest Physicians recommends treating only after positive imaging confirmation (Grade 1B) 1. Empiric heparin carries significant bleeding risk, particularly in a patient 6 weeks post-hip replacement with a large surgical site hematoma 2.
Why Not CT Venography (Option A)?
CT venography is reserved for situations where ultrasound is impractical (e.g., leg casting, excessive subcutaneous tissue preventing adequate compression assessment) or nondiagnostic 1. This patient has none of these limitations—the surgical site is clean, healing well, and there's no indication that ultrasound would be technically inadequate 1.
Ultrasound Protocol Specifications
The Society of Radiologists in Ultrasound recommends complete duplex ultrasound from the inguinal ligament to the ankle (including posterior tibial and peroneal veins in the calf), with compression performed at 2-cm intervals 1. This is superior to limited proximal-only protocols because:
- Limited protocols require repeat scanning in 5-7 days to safely exclude calf DVT propagation 1
- Whole-leg ultrasound is particularly appropriate for patients with severe symptoms (this patient has 2+ pitting edema of the entire leg) 1
- Post-surgical patients are at risk for both proximal and distal DVT 1
Special Considerations for Post-Hip Replacement Patients
Hip replacement surgery is a major risk factor for DVT, with thrombosis risk persisting for up to 2 months post-operatively 1. The presence of a large hematoma at the surgical site does not exclude DVT—in fact, research shows that 12% of patients undergoing ultrasound for suspected DVT have clinically significant incidental findings (including hematomas, pseudoaneurysms, and compartment syndrome), but DVT remains the most common serious finding requiring immediate treatment 3.
If Ultrasound is Positive
If proximal CUS or whole-leg ultrasound is positive for DVT, treatment should be initiated immediately without confirmatory venography (Grade 1B) 1. For provoked DVT following surgery, the American College of Chest Physicians recommends 3 months of anticoagulation (Grade 1B) 1.
If Ultrasound is Negative
If the initial ultrasound is negative but clinical suspicion remains high due to extensive leg swelling, the iliac veins should be imaged to exclude isolated iliac DVT, as standard compression ultrasound may miss this 1. Alternatively, repeat proximal ultrasound in 1 week or highly sensitive D-dimer testing can be performed (Grade 1B) 1.
Common Pitfalls to Avoid
- Do not delay imaging for D-dimer results in high-probability patients—this only postpones definitive diagnosis 1
- Do not assume the hematoma explains all findings—entire leg swelling suggests venous obstruction beyond local surgical trauma 1, 3
- Do not start anticoagulation empirically when diagnostic testing is available, especially with a large hematoma present that could expand with anticoagulation 1, 2
- Do not order limited proximal-only ultrasound in a patient with severe calf symptoms—whole-leg imaging is preferred 1