Venous thrombosis is a major cause of morbidity and mortality in the United States and a frequent cause of presentation in many emergency departments (EDs). The sequelae of deep vein thromboses (DVTs) range from the more common chronic venous stasis to the most serious pulmonary emboli (PEs).  PEs have been described as one of the most common preventable causes of death, and approximately two thirds of PEs are estimated to originate in the lower extremities as DVTs. The rate of propagation from DVT to PE is estimated to range from 10-50%. [2, 3, 4] Treatments with anticoagulation or Greenfield filter placement are extremely effective if used early, thereby underscoring the need for rapid diagnosis.
Compression ultrasonography has proven to be a highly sensitive and specific modality for the recognition of lower extremity DVTs without the need for radiation or contrast exposure. [5, 6] Traditional lower extremity studies interrogate and review the entire lower extremity vasculature, are performed by an ultrasonography technologist, and are read by a radiologist. [7, 8] However, these factors are not always available and have been shown to delay the time to diagnosis and potential treatment of a DVT by up to 2 hours. [9, 10]
To detect proximal lower-extremity DVTS, EDs now use a modified 2-point compression technique that focuses on the highest probability areas, decreases the study time to less than 5 minutes, and provides similar sensitivity and specificity. [11, 12, 13] In patients with a clinically suspected DVT, a negative compression ultrasound study may safely delay the need for anticoagulation therapy.  The 2-point DVT compression examination has been assessed in multiple randomized controlled studies and is well accepted when used properly with pretest probability assessments. [11, 15, 16]
In a study of limited-compression ultrasound (LC US), the standalone sensitivity and specificity of LC US were 91% (95% CI = 70-98%) and 97% (95% CI = 92-99%), respectively. According to the authors, incorporating LC US into the diagnostic approach would have reduced the rate of comprehensive imaging from 70% to 43%; D-dimer testing from 100% to 33%; and the mean time to diagnostic certainty by 5 hours. However, the investigators cautioned that although LC US holds promise as one component of the diagnostic approach to DVT, it should not be used as a standalone test because of imperfect sensitivity. 
In a systematic review and meta-analysis, Pomero et al compared emergency physician-performed ultrasound (EPPU) with color-flow duplex ultrasound (performed by a radiology department or vascular laboratory) and with angiography to diagnose DVT. The weighted mean sensitivity of EPPU compared to the reference imaging test was 96.1% (95% CI = 90.6-98.5%), and weighted mean specificity was 96.8% (95% CI = 94.6-98.1%). 
The safety, ease of use, rapid time to diagnosis, low cost, and accessibility make bedside ultrasonography for DVT especially useful for emergency and critical care clinicians.
Patients who have risk factors for DVT or pulmonary embolism (PE), and in whom a clinician suspects DVT or PE, should have workups that include, but are not necessarily limited to, bedside compression ultrasonography.
No absolute contraindications to bedside ultrasonography for DVT exist.
If the clinical suspicion and pretest probability for a PE are high enough that a spiral computed tomography (CT) scan with intravenous contrast or V/Q (ventilation/perfusion) scan is warranted, then ultrasonography should not delay such studies or any further treatment goals. Additionally, thrombus in the pelvic veins will not be detected with this technique and, although rare, may be best evaluated with CT or magnetic resonance (MR) venography. 
The following equipment is indicated:
Portable bedside ultrasound machine with a high-resolution linear transducer
The patient should be supine with the leg in question exposed up to the inguinal ligament. Bedside ultrasonography for deep vein thrombosis (DVT) is performed in 2 principal positions, one for each area of examination. The images below depict ideal positions. Patient status and cooperation, however, ultimately determine what kind of positioning is possible. Ideally, 30-40 degrees of reverse Trendelenburg facilitates the examination by increasing venous distention.
When examining the femoral vein, the patient should be supine with the hip externally rotated and flexed, as shown below.
When examining the popliteal vein, the patient needs to expose the popliteal fossa on the posteromedial aspect of the knee. The patient can either dangle the leg off the edge of the bed or bend the knee and externally rotate the hip, as shown below. If necessary, the patient can also be rolled onto his or her side or into the prone position.
Set up the portable ultrasound machine at the patient's bedside, with the linear transducer set at a frequency of 5.0-7.0 MHz. Expose the patient’s entire leg.
By convention, clinicians in EDs scan in an abdominal orientation with the probe marker pointed toward the patient’s head or right side. During the bedside ultrasonographic examination for deep vein thrombosis (DVT), the technician maintains this convention; that is, the probe marker points toward the patient’s right. In terms of orientation, remember that the top of the viewing screen is always where the transducer is touching the patient. Vascular anatomy is shown in the image below.
The video below demonstrates ultrasonographic evaluation for DVT.
Position the patient as noted previously for examination of the femoral vessels (see Positioning, above). The study begins with an examination of the common femoral vein just distal to the inguinal ligament. The femoral vessels are located just inferior to the inguinal ligament and approximately midway between the pubic symphysis and the anterior superior iliac spine. The femoral artery is usually palpable. This is the initial point of examination.
Apply gel to the transducer, the patient’s leg, or both, and position the transducer transversely, just distal to the inguinal ligament, as shown below. Remember, the indicator on the probe should point toward the patient’s right. In this transverse view, the vein is imaged in cross-section.
Drag or fan the transducer in a cephalad or caudad direction until the junction of the common femoral vein and the greater saphenous vein can be visualized, as shown below. The common femoral artery is lateral to the common femoral vein.
Using the transducer, apply direct pressure to completely compress the vein. If the vein compresses completely, then a DVT at this site can be ruled out.
Be sure that enough pressure is being applied and being applied evenly. Apply enough pressure so that slight deformation of the artery is noticeable. If the vein is still not completely compressible, a DVT is present. See the image below.
Complete compression of the vein rules out a DVT, whereas the inability to completely compress the vein rules in a DVT. Thus, compressibility is the rule in/rule out criterion for DVT on ultrasound. (See Results, below, for more details.)
Compressibility must be present in both the femoral veins and the popliteal vein. Sometimes, the angle of the transducer may need to be adjusted in order to completely compress the vein. The greater saphenous vein is a superficial vein. A clot in the greater saphenous vein near its junction with the common femoral vein, however, can easily propagate.
The examination of the common femoral vein should extend from 2 cm proximal to 2 cm distal to the intersection of the common femoral and greater saphenous veins. Distal to the greater saphenous vein, the common femoral vein splits into the deep and superficial femoral veins.
Despite its name, the superficial femoral vein is indeed a deep vein. Once collapse of both the deep and superficial femoral veins is confirmed, the examination may move on to the popliteal vein.
Position the patient as noted earlier for examination of the popliteal vessels. (See Positioning, above.)
Again, apply gel to the transducer, the patient’s leg, or both, and position the transducer transversely in the popliteal fossa, with the probe marker directed toward the patient’s right, as shown below.
Drag or fan the transducer in a cephalad or caudad direction until the superficial popliteal artery and vein are visible, as shown below. The popliteal vein is usually posterior to the popliteal artery. Given the posterior approach of the probe (transducer face is placed in the popliteal fossa), however, the vein appears more superficial (closer to the transducer face) than the artery. The popliteal vessels are compressed more easily, so reducing probe pressure may help visualize the veins.
The examination should include the distal 2 cm of the popliteal vein and the proximal aspects of its trifurcation into the anterior tibial vein, the posterior tibial vein, and the peroneal vein. Anatomic variability is not uncommon, and the popliteal vein is often seen dividing into the anterior and posterior tibial veins, with the peroneal vein then splitting off from the posterior tibial vein.
Although not a formal component of the focused lower extremity compression examination for DVT, Doppler ultrasonography may be useful to help determine anatomic orientation and to further interrogate potentially misleading structures. Information obtained from Doppler ultrasonography alone, however, does not yield definitive evidence regarding clot presence.
Doppler examination assesses the direction, velocity, and pattern of blood flow, with venous and arterial vessels demonstrating characteristic patterns. Normal venous vasculature should show venous flow at baseline, augmentation of flow with calf compression, and phasic respiratory ventilation with increased flow during expiration. In general, augmentation helps to assess for obstruction distal to the probe, whereas respiratory variation helps to assess for obstruction proximal to the probe (ie, iliac veins and inferior vena cava).
Some points to keep in mind include the following:
Vessel filling defects may indicate on-site or upstream flow obstruction
Cysts or other fluid cavities are devoid of flow
Lymph nodes demonstrate dense, high vascularity
For the focused deep vein thrombosis (DVT) compression ultrasonographic examination, complete compressibility is the only absolute criterion to rule out DVT.
Lack of compressibility is the only absolute criterion to rule in DVT. Documentation of the most proximal aspect of the clot is important to assess progression or regression of the clot after intervention.
Direct visualization of a clot and Doppler flow abnormalities may suggest a DVT and, if clinically appropriate, may justify serial scans. Alone, however, these studies are limited in terms of their ability to definitively diagnose a DVT. See the image below.
The videos below depict an absence of DVT in the femoral and popliteal veins, respectively.
Bedside ultrasonography for assessment of DVT poses no significant complications. No evidence exists for the potential propagation of clot through leg manipulation or vessel compression.