01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Compression ultrasound combines B-mode imaging, graded compression and Doppler. A normal vein is thin-walled and collapses under appropriate pressure; acute thrombus makes the vein partly or completely non-compressible and may expand its lumen. Echogenicity, colour filling and spectral phasicity support the diagnosis but are not substitutes for technically adequate compression in compressible segments. The report should name each segment, laterality, thrombus extent, occlusive status and limitations.
The word negative has meaning only when paired with protocol and clinical context. Limited proximal CUS may examine two or three territories, usually the common femoral and popliteal veins with femoral-vein assessment depending on the protocol, so the report must identify the actual segments examined. Whole-leg ultrasound extends through calf axial and muscle veins. Pelvic veins and the IVC may remain poorly visualised. A negative limited study can sit within a validated serial pathway, while suspected calf disease calls for whole-leg ultrasound and an inconclusive study or unresolved pelvic or IVC question may require cross-sectional venography.
Acute and chronic appearances overlap. Acute thrombus is often hypoechoic, vein-expanding and poorly compressible; chronic post-thrombotic change may be echogenic, wall-adherent, contracted, recanalised or associated with collaterals. Ultrasound cannot always date thrombus reliably. Compare previous imaging and ask whether findings are new before labelling recurrent ipsilateral DVT and exposing the patient to another anticoagulant course.
Key points
- Do not let routine ultrasound delay resuscitation or vascular escalation for shock, suspected pulmonary embolism, or a blue, severely painful, rapidly swollen limb.
- After an adequate negative limited proximal study, repeat timing depends on the pathway: NICE uses 6–8 days for non-pregnant likely DVT with positive D-dimer, whereas RCOG uses repeat scans on days 3 and 7 in pregnancy when suspicion remains high.
- Limited proximal CUS samples two or three named venous territories—usually common femoral and popliteal, with femoral-vein assessment depending on protocol—rather than a universal continuous segment. The report must name what was examined; unresolved calf disease calls for whole-leg ultrasound, while suspected pelvic or IVC disease or inconclusive ultrasound may require CT or MR venography.
- Failure of a normally compliant vein to collapse with graded probe pressure is the central direct sign of DVT; colour-flow absence alone can reflect slow flow or technical settings.
- When calf DVT is specifically suspected and whole-leg ultrasound is available, ESVS recommends whole-leg imaging; a technically limited examination is incomplete rather than negative.
- Suspected iliac or caval thrombosis with an inadequate or negative leg scan may require CT or MR venography selected with radiology and renal, contrast and radiation risks considered.
- In pregnancy, use the obstetric pathway: RCOG recommends compression duplex ultrasound and, when initial imaging is negative but suspicion stays high, repeat scans on days 3 and 7.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Partial or complete failure of venous collapse under graded pressure, with compatible intraluminal material, supports acute DVT in that named segment.
Thrombus in popliteal, femoral or iliac territory is proximal DVT and generally changes anticoagulation management promptly.
Thrombus confined to axial calf or gastrocnemius and soleal veins is distal DVT and needs explicit symptom, extension and bleeding-risk assessment.
Whole-leg swelling, groin or back pain, abnormal common femoral phasicity or prominent collaterals may indicate pelvic disease outside routine compression views.
Echogenic wall thickening, narrowed irregular channels, synechiae and collateralisation can represent old disease rather than a new acute clot.
Pain, obesity, oedema, wounds, dressings, plaster and deep pelvic anatomy can prevent adequate visualisation or compression and must be reported.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Proximal compression ultrasoundFirst step - Why
- Assess two or three named proximal venous territories rapidly, usually common femoral and popliteal with femoral-vein assessment depending on protocol.
- Interpretation and limitations
- Non-compressibility confirms thrombus in the named segment. A negative result excludes only adequately visualised proximal territory and must be interpreted with probability and D-dimer.
- 02
Whole-leg ultrasound - Why
- Examine proximal, axial calf and muscular calf veins in one session.
- Interpretation and limitations
- Use when isolated calf DVT is suspected and expertise is available. It can avoid serial scanning but may identify distal thrombi of uncertain consequence, so management still needs risk assessment.
- 03
Repeat proximal ultrasound - Why
- Detect proximal extension after an initially negative proximal examination.
- Interpretation and limitations
- NICE specifies 6–8 days after likely DVT with negative proximal imaging and positive D-dimer. This is a conditional branch, not a universal repeat rule.
- 04
CT venography - Why
- Assess pelvic, iliac or caval veins and identify obstructing pelvic pathology.
- Interpretation and limitations
- Useful when ultrasound is infeasible or inconclusive, but iodinated contrast, renal function and radiation exposure must be considered and imaging must not delay phlegmasia treatment.
- 05
MR venography - Why
- Provide non-ionising problem-solving imaging for pelvic or IVC thrombosis when ultrasound is inappropriate, infeasible or inconclusive.
- Interpretation and limitations
- MR venography can assess pelvic veins, the IVC and causes of venous obstruction, but availability, time, contraindications and contrast issues limit emergency use. Distinguishing selected suspected recurrent ipsilateral DVT from persistent thrombus is a different application of magnetic resonance direct thrombus imaging, not routine MR venography.
- 06
Previous venous imaging - Why
- Establish whether current abnormalities are new, extended or chronic.
- Interpretation and limitations
- Unchanged residual non-compressibility supports chronic disease; a new segment or increased non-compressible diameter supports recurrence when matched to current symptoms.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked case: likely DVTInterpret a negative proximal studyFirst stepA non-pregnant adult has Wells score 3, negative adequate proximal ultrasound and positive D-dimer.+
- 1Verify that this case report explicitly documents the common femoral, femoral and popliteal territories stated in the stem and records no technical limitation; other limited protocols may examine only two named territories.
- 2Stop interim therapeutic anticoagulation while preserving any unrelated established long-term anticoagulant indication.
- 3Arrange repeat proximal ultrasound for 6–8 days and give immediate return advice for progression, PE symptoms or limb colour change.
- 4If the repeat scan is positive, treat confirmed proximal DVT; if negative, close the DVT pathway, review alternatives and document the endpoint.
02Suspected distal diseaseSelect whole-leg imagingCalf-localised symptoms persist and the initial proximal examination did not assess calf veins.+
- 1Confirm that symptoms localise below the knee and that the previous report was proximal rather than whole-leg.
- 2Request whole-leg ultrasound where available, ensuring axial and muscular calf veins and technical adequacy are described.
- 3If distal DVT is confirmed, assess symptoms, extension risk, provoking factors, cancer and bleeding risk before choosing treatment or surveillance.
- 4If surveillance is chosen, arrange clinical reassessment and repeat whole-leg ultrasound after one week as ESVS recommends.
03Central-vein problem solvingEscalate beyond routine compressionEscalationThe entire leg is swollen, iliac thrombosis is suspected, or ultrasound is repeatedly inadequate or discordant.+
- 1Discuss the unresolved anatomical question with vascular imaging and review renal function, contrast allergy, pregnancy and urgency.
- 2Choose CT venography for rapid central mapping or MR venography when its advantages outweigh delay and access limitations.
- 3EscalationEscalate immediately for phlegmasia or threatened limb; cross-sectional imaging must support rather than postpone treatment.
04Pregnancy pathwayUse obstetric serial imagingA pregnant or puerperal patient has suspected DVT and an initial negative compression duplex scan.+
- 1Do not use the general Wells/D-dimer exclusion rule; reassess the clinical level of suspicion with the obstetric team.
- 2If suspicion is low, anticoagulation may be discontinued after negative ultrasound according to RCOG.
- 3If suspicion remains high, discontinue anticoagulation and repeat compression ultrasound on days 3 and 7; restart and continue treatment if DVT is then confirmed.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Document the exact venous segments examined, compression response, Doppler findings, thrombus extent and every technical limitation.
- Track whether a planned 6–8-day proximal repeat or pregnancy day-3 and day-7 repeat was completed and acted upon.
- Compare recurrent ipsilateral symptoms with previous images before diagnosing a new event in chronically abnormal veins.
- After a negative pathway, reassess symptoms and pursue cellulitis, Baker cyst, haematoma, lymphoedema, superficial thrombosis or musculoskeletal causes.
- Escalate new chest symptoms, haemodynamic instability, cyanosis, severe pain, sensory loss or weakness immediately.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Compression outranks colour
Absent colour can arise from low flow or settings, whereas appropriate failure of venous collapse directly demonstrates luminal obstruction in accessible segments.
Name the negative territory
A negative proximal scan and a negative whole-leg scan answer different anatomical questions, so reports and decisions must state the protocol.
Calf detection has consequences
Whole-leg imaging finds distal thrombi that proximal protocols deliberately manage through serial assessment; discovery does not create one universal anticoagulation rule.
Pelvic veins hide
Routine leg compression may miss iliac or caval thrombosis, particularly with entire-leg swelling, back pain or abnormal central flow signals.
Dating remains uncertain
Acute and chronic ultrasound appearances overlap, so previous imaging and interval anatomical change are central when recurrent ipsilateral DVT is suspected.
07Common pitfallsFrequent interpretation and management errors.
- 01
Reporting a technically limited examination as negative without naming the unvisualised territory.
- 02
Treating normal proximal compression as exclusion of isolated calf, iliac or caval DVT.
- 03
Using absent colour flow alone to diagnose occlusion without compression and technical optimisation.
- 04
Calling residual wall thickening a new acute DVT without prior imaging or interval change.
- 05
Repeating every negative scan automatically instead of following the probability, D-dimer and population-specific branch.
- 06
Applying the non-pregnant 6–8-day repeat rule to a pregnancy pathway that specifies days 3 and 7.