The test is COMPRESSION, not colour. A vein that fully collapses under the probe has no clot in it.
Orient first
- The primary diagnostic manoeuvre is compressibility in TRANSVERSE. A normal vein collapses completely until its walls touch; a vein containing thrombus does not.
- Colour and spectral Doppler are supportive, not primary. Fresh thrombus can be anechoic and invisible on grey-scale, which is exactly why you press rather than look.
- Transverse, not longitudinal: in longitudinal the probe slides off the vein and you get a false impression of collapse.
Acquire the study
- Linear probe for the leg; curvilinear for a large thigh or the iliac veins.
- Patient supine with the leg externally rotated and slightly flexed for the femoral and popliteal segments; sitting or prone helps the calf veins fill.
- Compress every 2 cm along the vein, in TRANSVERSE, using enough pressure to just deform the accompanying artery.
- Set Doppler for low velocity when assessing venous flow, augmentation and respiratory phasicity.
The manoeuvre
- Start at the common femoral vein at the groin, including the saphenofemoral junction.
- Work down: common femoral, femoral vein through the adductor canal, popliteal vein, then the calf trifurcation and the paired calf veins.
- At each level compress until the walls appose. Record any segment that does not fully compress.
- For each thrombus, state the extent and its most PROXIMAL level — that is what determines management.
- Describe the thrombus: occlusive or non-occlusive, and whether it appears acute (expanded, anechoic, spongy) or chronic (contracted, echogenic, retracted with collaterals).
- Test flow: spontaneous, phasic with respiration, and augmenting on distal calf compression. Loss of phasicity suggests a more central obstruction ABOVE the level you can see.
- Look for alternative causes of the swollen leg: a Baker cyst, a haematoma, cellulitis, a superficial thrombophlebitis, or a mass compressing the vein.
What confirms it
- A non-compressible venous segment. That is the diagnosis.
What licenses you to exclude it
- Full compressibility of every interrogated segment excludes thrombus in those segments.
- ⚠️ Say WHICH segments were assessed. A study that did not include the calf does not exclude calf DVT, and one that could not see the iliacs does not exclude pelvic thrombus.
- ⚠️ Non-phasic flow with a compressible visible venous system suggests obstruction PROXIMAL to the field of view — that is a positive finding requiring further imaging, not a normal study.
The classic misread
- Assessing compression in longitudinal, where the probe slides off the vein.
- Relying on colour Doppler and missing anechoic acute thrombus.
- Not stating which segments were examined.
- Missing a duplicated femoral vein and clearing only one of the two channels.