What is the recommended test for diagnosing a blood clot in the leg?

Diagnostic Testing for Blood Clots in the Leg

Compression ultrasound (CUS) is the recommended first-line diagnostic test for suspected deep vein thrombosis (DVT) in the leg, with the choice between proximal CUS or whole-leg ultrasound depending on clinical circumstances. 1

Diagnostic Algorithm Based on Pretest Probability

Low Pretest Probability

• Begin with a highly sensitive D-dimer test; if negative, no further testing is needed 1, 2
• If D-dimer is positive, proceed to proximal compression ultrasound 1

Moderate Pretest Probability

• Either start with a highly sensitive D-dimer or proceed directly to ultrasound 1, 2
• If D-dimer is negative, DVT is excluded 1
• If D-dimer is positive, proximal compression ultrasound is recommended 1

High Pretest Probability

• Proceed directly to proximal compression ultrasound (CUS) or whole-leg ultrasound 1, 2
• D-dimer testing alone is not recommended to rule out DVT in high pretest probability patients 1

Types of Ultrasound Testing

• Proximal Compression Ultrasound (CUS): Examines common femoral and popliteal veins; sensitivity 93% and specificity 98% for proximal DVT 1, 3
• Whole-leg Ultrasound: Examines entire leg including calf veins; preferred for patients unable to return for serial testing or with severe symptoms consistent with calf DVT 1
• Combined Modality Ultrasound: Compression with either Doppler or color Doppler; recommended for suspected upper extremity DVT 1

Follow-up Testing Recommendations

• If initial proximal CUS is negative but clinical suspicion remains:

  • Repeat proximal CUS in 1 week, or
  • Perform D-dimer testing, or
  • Perform whole-leg ultrasound 1

• If initial proximal CUS is negative but D-dimer is positive:

  • Repeat proximal CUS in 1 week or perform whole-leg ultrasound 1

• If whole-leg ultrasound is negative:

  • No further testing is needed 1

Special Considerations

• For patients with extensive unexplained leg swelling and negative proximal CUS, the iliac veins should be imaged to exclude isolated iliac DVT 1
• In pregnant patients with suspected DVT, initial evaluation with proximal CUS is recommended over D-dimer testing 1, 2
• For suspected upper extremity DVT, initial evaluation with combined modality ultrasound (compression with either Doppler or color Doppler) is recommended 1
• When ultrasound is impractical (e.g., leg casting, excessive subcutaneous tissue) or nondiagnostic, CT venography, MR venography, or MR direct thrombus imaging can be used as alternatives 1

Clinical Pearls and Pitfalls

• Bedside diagnosis of DVT without imaging is highly inaccurate; only about 30% of clinically suspected cases are confirmed by ultrasound 4
• Ultrasound can identify other causes of leg pain or swelling in 5-15% of cases, including pseudoaneurysm, arterial occlusive disease, compartment syndrome, and tumors 5, 3
• D-dimer may be falsely elevated in pregnancy, inflammation, malignancy, and advanced age, making ultrasound a better initial test in these populations 2
• Emergency physician-performed ultrasound has shown high accuracy (sensitivity 96.1%, specificity 96.8%) and may be useful when radiology services are not immediately available 6

Ultrasound is safe, accurate, efficient, and non-invasive, making it the diagnostic modality of choice for suspected DVT in symptomatic patients 4, 3.