Duplex Ultrasound (DUS) vs. Venous Compression Test (VCT)
Key Distinction
Venous Compression Test (VCT) is actually a component technique within Duplex Ultrasound (DUS), not a separate imaging modality—DUS combines compression ultrasound with Doppler flow analysis to provide comprehensive venous evaluation. 1
Understanding the Terminology
Duplex Ultrasound (DUS) Components
DUS integrates two complementary techniques:
- Compression ultrasound (the "venous compression test"): Real-time B-mode imaging with manual probe pressure to assess vein compressibility 1
- Doppler imaging: Spectral, color-flow, or continuous-wave Doppler to evaluate blood flow patterns 1
The compression component is considered the more definitive diagnostic criterion, while Doppler provides supplementary hemodynamic information. 1
What "Compression Ultrasound" or VCT Actually Means
When clinicians refer to "compression ultrasound" or "venous compression test" alone, they typically mean performing only the B-mode compression assessment without Doppler analysis—essentially performing half of a duplex examination 1
How Venous Compression Testing is Performed
Technical Execution
The sonographer applies graded external pressure with the ultrasound transducer perpendicular to the vein while visualizing it in real-time B-mode imaging. 1
Key technical steps include:
- Transverse (short-axis) imaging: The vein is visualized in cross-section, appearing as a circular structure 1
- Sequential compression: Pressure is applied at multiple anatomic levels along the venous system 1
- Systematic evaluation: Examination proceeds from common femoral vein through popliteal vein, with extension to calf veins when indicated 2, 3
Anatomic Coverage
A comprehensive examination should include:
- Deep venous system (common femoral, femoral, popliteal, and calf veins) 2, 3
- Great saphenous vein (GSV) and small saphenous vein (SSV) 2
- Perforating veins when evaluating chronic venous insufficiency 2
Examining only the common femoral and popliteal veins misses 30.3% of all DVT cases, including isolated superficial femoral vein and calf vein thrombosis. 3
Result Interpretation
Primary Diagnostic Criterion
Complete compressibility of the vein walls with applied pressure indicates absence of thrombus—failure of complete compression is the definitive sign of DVT. 1
Positive Findings (DVT Present)
- Non-compressibility: Vein walls fail to coapt completely under pressure 1
- Visible echogenic material: Direct visualization of thrombus within the vein lumen 2
- Vein distension: Affected vein appears dilated compared to contralateral side 4
Negative Findings (No DVT)
- Complete compression: Vein walls touch completely, obliterating the lumen 1
- Normal vein caliber: Appropriate size relative to adjacent artery 4
Diagnostic Performance
For proximal DVT (femoral and popliteal veins), compression ultrasound demonstrates:
For distal (calf) DVT, sensitivity drops significantly to 59.8-67.0% (pooled 63.5%), representing a major limitation. 1
Underlying Physiology
Why Compression Works
Normal veins are thin-walled, low-pressure structures that collapse completely under external pressure—thrombus prevents this collapse by providing structural resistance within the lumen. 1
Physiologic principles:
- Venous compliance: Normal veins have high compliance and low intraluminal pressure (5-15 mmHg), allowing easy compression 4
- Thrombus resistance: Acute thrombus is semi-solid and incompressible, preventing vein wall apposition 1, 2
- Chronic changes: Older, organized thrombus may partially recanalize, making compression assessment less reliable 1, 4
When Doppler Adds Value
Doppler flow analysis provides supplementary information when compression is technically limited or results are equivocal:
- Flow augmentation: Distal limb compression should produce increased venous flow velocity 1
- Respiratory phasicity: Normal central veins show flow variation with breathing 4
- Characterizing partial occlusion: Color Doppler can identify flow around non-occlusive thrombus 1
However, augmentation of venous flow rarely provides additional diagnostic information beyond compression and should be considered a secondary tool. 1
Clinical Limitations and Pitfalls
Technical Limitations of Compression-Only Testing
Compression ultrasound has reduced accuracy in specific anatomic locations:
- Central veins: Cannot adequately compress pelvic veins, proximal subclavian vein, IVC, or SVC 1, 2
- Calf veins: Difficult to visualize and compress, with sensitivity only 63.5% 1
- Obese patients: Increased tissue depth limits compression effectiveness 1, 2
For suspected iliocaval or upper extremity central venous thrombosis, CT venography or MR venography should be performed instead. 1, 2
When Full Duplex is Essential
Adding Doppler to compression becomes critical when:
- Bandages, casts, or pain limit adequate compression 1, 2
- Distinguishing acute from chronic DVT: Flow patterns and recanalization channels help differentiate 1, 4
- Evaluating upper extremity DVT: Combined modality ultrasound (compression plus Doppler) is recommended over compression alone 1
Common Diagnostic Errors
Pitfalls to avoid:
- Inadequate compression pressure: Insufficient force fails to collapse normal veins, creating false positives 4
- Examining only CFV and popliteal vein: Misses 30.3% of DVT cases 3
- Operator dependency: Compression technique requires expertise and is highly operator-dependent 1, 4
Practical Clinical Algorithm
For suspected lower extremity DVT:
- Initial test: Full duplex ultrasound (compression + Doppler) of entire deep venous system 1, 2
- If negative with high clinical suspicion: Repeat duplex in 1 week or proceed to CT/MR venography 1
- If positive: Compression findings alone are sufficient to initiate anticoagulation 5
For suspected upper extremity DVT:
- Initial test: Combined modality ultrasound (compression + Doppler) 1
- If negative with high suspicion: CT venography or MR venography for central veins 1
For isolated calf DVT with anticoagulation contraindications: