Clinicians who search for duplex venous ultrasound usually want a working protocol, not a consumer overview. The exam combines grayscale imaging with color and spectral Doppler so you can document vein anatomy, compressibility, and flow in one study. It is the first-line test for suspected lower-extremity deep vein thrombosis (DVT) and the standard method for evaluating chronic venous disease and superficial reflux. This article reviews transducer choice, positioning, a complete lower-extremity protocol, DVT and reflux criteria, how acute thrombus differs from chronic change, and the pitfalls that produce false-positive or false-negative results.
What Duplex Adds Compared With B-mode or Doppler Alone
B-mode ultrasound shows anatomy and allows compression. Doppler, whether color or spectral, shows flow direction, phasicity, and velocity. Duplex means both are used together. A grayscale-only study can diagnose occlusive DVT by noncompressibility. Duplex adds flow information that helps confirm patency, detect partial obstruction, assess iliac inflow, and quantify reflux. In ordering language, venous Doppler, venous duplex, and venous ultrasound are often used as if they were the same test. They are not.
Vascular ultrasound is the broader category. It includes venous and arterial duplex of the extremities as well as mesenteric, renal, and extracranial carotid studies. Venous duplex asks whether the vein is patent, compressible, and competent. Arterial duplex asks whether there is stenosis, occlusion, or an abnormal waveform. Protocols, Doppler presets, and diagnostic criteria are not interchangeable.
Indications and What the Exam Should Answer
Typical indications include suspected DVT, unexplained unilateral swelling or pain, follow-up of known thrombus, chronic venous insufficiency, preoperative superficial mapping, and venous ulcer evaluation. Match the protocol to the question. A focused DVT study and a standing reflux map are related but not the same examination. The report should answer:
- Is there acute DVT, and which segments are involved?
- Is thrombus occlusive or nonocclusive, and does it appear acute, chronic, or indeterminate?
- Is there superficial thrombosis?
- If reflux is the indication, which deep, superficial, and perforator segments are incompetent, and what is the reverse-flow duration?
- Are there variants, such as duplicated femoral veins, that affect interpretation?
Equipment, Positioning, and Transducer Choice
Use a high-frequency linear transducer, commonly in the 5 to 12 MHz range, for femoral, popliteal, and calf veins. Switch to a lower-frequency linear or curved transducer for obese or edematous limbs and for iliac veins or the inferior vena cava when those segments are included. Optimize grayscale so vein walls and lumen are sharp. For color and spectral Doppler, use a low wall filter, low pulse repetition frequency, and a small color box. Steer the box and use heel-toe angling. Venous velocities are low, so arterial presets will underfill veins.
For DVT evaluation, position the patient supine or in reverse Trendelenburg, which distends the veins and makes noncompressibility easier to judge. Externally rotate the hip and flex the knee slightly to open femoral and popliteal windows. For reflux, standing is preferred when the patient can tolerate it. If standing is not feasible, reverse Trendelenburg with provocative maneuvers is an accepted alternative. Keep the room warm, because cold veins constrict and can mimic disease. Note laterality, prior DVT, recent surgery, and anticoagulation, which change how residual thrombus is described.
Duplex Venous Ultrasound Protocol for the Lower Extremity
A complete lower-extremity exam is systematic, segment by segment, with compression in the transverse plane and Doppler in the longitudinal plane. Compress the vein every 1 to 2 cm with enough pressure to coapt the walls while the adjacent artery remains open. If the vein does not collapse fully, assume thrombus until you prove otherwise. Superficial mapping can be added or performed as a dedicated study.
Deep veins
- Start at the groin in transverse. Identify the common femoral vein (CFV), common femoral artery, and saphenofemoral junction. Compress the CFV above and below the junction.
- Follow the CFV to the confluence of the femoral vein (FV) and profunda femoris vein. Compress both origins. Duplicated femoral veins are common. Image both channels or you will miss isolated thrombus.
- Track the FV through the thigh in transverse, compressing continuously, then sample Doppler in longitudinal at proximal, mid, and distal thigh.
- Move to the popliteal fossa. Compress the popliteal vein, usually superficial to the artery in this window, from the adductor hiatus through the trifurcation.
- Evaluate the paired posterior tibial and peroneal veins. Include gastrocnemius and soleal veins when calf symptoms are present. Assess anterior tibial veins if the clinical picture points to the anterior compartment.
- If iliac or inferior vena cava thrombosis is a concern, or if the CFV waveform is abnormal, extend the study proximally with a lower-frequency transducer.
Record representative compression images or cine loops at each named segment, plus color and spectral Doppler at the CFV, FV, and popliteal vein. Compare the symptomatic limb with the contralateral CFV waveform when iliac obstruction is possible. For structured case review of thrombus diagnosis, see IAME's course on duplex diagnosis of lower-extremity venous thrombosis.
Superficial veins and reflux mapping
When the indication is venous insufficiency, varicose veins, or ulceration, map the great saphenous vein (GSV), small saphenous vein (SSV), accessory saphenous veins, and relevant perforators. Measure diameter at standard locations, note tributaries that feed varicosities, and test each segment for reverse flow with distal compression or a Valsalva maneuver. Standing or reverse Trendelenburg positioning is essential. A dedicated ultrasound assessment and mapping of the superficial venous system is the right framework when the clinical question is reflux rather than acute DVT.
Color and spectral Doppler
Color Doppler confirms lumen fill and direction. Spectral Doppler at the CFV should show respiratory phasicity. Distal augmentation should produce a brisk antegrade signal if the vein is patent between the sample volume and the calf. Continuous, nonphasic CFV flow, especially if asymmetric, raises concern for more proximal obstruction. Do not rely on color fill alone. Slow flow, suboptimal settings, and a perpendicular beam can leave a patent vein without color.
Avoid vigorous distal augmentation once you have seen free-floating or loosely attached thrombus. Gentle documentation of spontaneous flow is enough. Practical vascular ultrasound techniques for better results apply here: optimize the angle, keep the color box small, and match the scale to venous velocities.
Noncompressibility is the primary criterion for DVT. Color fill and spectral waveforms support the diagnosis but do not replace a technically adequate compression maneuver.
Diagnostic Criteria for Acute DVT
Noncompressibility of the vein is the primary diagnostic criterion. Supportive findings strengthen the call but do not replace compression.
- Noncompressibility: the vein walls do not coapt with probe pressure sufficient to deform the adjacent artery.
- Intraluminal material: acute thrombus is often hypoechoic or nearly anechoic and can be missed if you look only for an obvious clot.
- Vein distention: an acutely occluded vein is typically enlarged and round rather than flattened.
- Absent or reduced color fill in a technically adequate image, with a filling defect or a blunted spectral signal.
- Loss of phasicity or augmentation when obstruction lies between the sample site and the heart or the calf, respectively.
Partial compressibility with a residual lumen suggests nonocclusive thrombus. Always document the most proximal extent. Isolated calf DVT is real. If the calf veins are not examined, say so in the report rather than implying a complete negative study.
Acute Versus Chronic Deep Vein Thrombosis
Age of thrombus is a common source of overcalls and undercalls. Use a cluster of findings. No single feature is definitive, and mixed or indeterminate appearance is acceptable when the age is unclear.
Findings that favor acute DVT:
- Enlarged, smooth-walled vein
- Homogeneous hypoechoic or anechoic lumen
- Spongy deformation rather than a rigid cord
- Poorly attached or free-floating thrombus
- Absent collaterals and no recanalization channels
Findings that favor chronic postthrombotic change:
- Normal or reduced vein caliber
- Echogenic, irregular, or thickened walls
- Synechiae, bands, or recanalized channels
- Firm, attached material that does not deform easily
- Collateral veins and reflux in a recanalized lumen
A previously thrombosed vein may be scarred yet patent. New symptoms with a larger, less compressible, more hypoechoic segment superimposed on chronic change suggest acute-on-chronic thrombus. Compare with prior studies whenever they exist. If you cannot age the thrombus, describe the findings and state that the age is indeterminate.
Reflux Criteria
Reflux is reverse flow after a provocative maneuver, measured with spectral Doppler. Commonly cited cutoffs from society consensus statements are approximately greater than 0.5 second in the superficial veins, greater than 1.0 second in the femoropopliteal deep veins, and greater than 0.35 second in perforators. Sources differ slightly, so report the measured duration rather than only a binary label, and use the threshold your laboratory has adopted.
Test the saphenofemoral junction, GSV trunk, SSV, and deep veins at the CFV and popliteal vein at a minimum. Document large, refluxing perforators near an ulcer or varicosity, including diameter and location. Diameter alone does not diagnose reflux. A large vein that does not reverse flow is not incompetent by duplex criteria.
Pitfalls That Cause False-Positive and False-Negative Studies
Most errors are technical. Run a short checklist before you call the study normal or positive.
- Incomplete compression: inadequate pressure, a steep angle, or bone in the way can mimic thrombus. Reposition and compress until the artery deforms.
- Duplicated femoral or popliteal veins: thrombus in the unimaged channel produces a false-negative study.
- Anechoic acute thrombus: looking for an echogenic filling defect instead of performing compression misses acute DVT.
- Slow flow mistaken for thrombus: increase gain, drop the scale, and use augmentation when safe before calling occlusion.
- Iliac DVT missed at the groin: a compressible CFV does not exclude iliac or IVC thrombus. Use CFV waveform asymmetry and proximal imaging.
- Calf veins omitted: isolated tibial, peroneal, or muscular vein DVT will be missed if the protocol stops at the popliteal vein.
- Reflux tested recumbent only: incompetence can be underestimated without standing or reverse Trendelenburg.
- Wrong Doppler settings: a high wall filter, high PRF, or a 90-degree beam creates an empty lumen in a patent vein.
- Chronic scarring called acute DVT: use caliber, echogenicity, wall thickening, and collaterals, and compare with prior exams.
If the study is limited by wounds, casts, obesity, or inability to reposition, state the limitation and which segments were not seen.
Reporting Pearls
Name each segment as patent and compressible, acute DVT, chronic postthrombotic change, or indeterminate. Give the most proximal extent of thrombus. Separate deep from superficial findings. For reflux studies, list each incompetent segment with reverse-flow duration and diameter when relevant. Avoid vague phrases that only say the Doppler was negative without stating what was compressed and sampled. Clinical decisions depend on location and chronicity, not on a single positive or negative label.
Duplex venous ultrasound is accurate when compression is thorough, Doppler settings match venous flow, and acute thrombus is distinguished from chronic change. A protocol-first approach, with attention to duplicated veins, the calf, and the CFV waveform, prevents most false results. When the question is reflux, add standing or reverse Trendelenburg mapping of the superficial system. IAME offers accredited ultrasound CME courses on duplex diagnosis of lower-extremity venous thrombosis and on superficial venous mapping for clinicians who want structured case review.
Frequently Asked Questions
How do you perform a venous duplex ultrasound?
Use a high-frequency linear transducer and compress each deep vein in transverse from the groin through the calf, then sample color and spectral Doppler in longitudinal. Position the patient in reverse Trendelenburg for DVT and standing or reverse Trendelenburg for reflux. Document noncompressibility, thrombus appearance, waveforms, and reverse-flow duration when indicated. Extend the study proximally if the common femoral waveform is continuous or asymmetric.
What is the difference between an ultrasound and a duplex scan?
Ultrasound in this context often means grayscale B-mode imaging. Duplex adds Doppler, so you see both structure and flow. A compression-only grayscale exam can diagnose occlusive DVT, while duplex is needed to assess partial obstruction, proximal inflow, and valvular reflux. In practice, most venous ultrasounds ordered for DVT are duplex studies.
How much does duplex ultrasound cost?
Charges vary by facility, region, and insurance coverage, so there is no single price. Patients and referring clinicians should confirm current fees with the imaging site and the payer. This article does not address billing or coding.
What are the key differences between acute and chronic deep vein thrombosis on ultrasound?
Acute DVT typically enlarges the vein, looks hypoechoic or anechoic, and may be spongy or poorly attached, without collaterals. Chronic change usually shows a normal or small caliber vein, echogenic thickened walls, synechiae or recanalized channels, attached material, and collaterals. Mixed features are common. If the age is unclear, report the findings as indeterminate rather than forcing a binary call.
