The strategy of good clinical risk stratification and the appropriate use of the D-dimer assay is effective in excluding the diagnosis in approximately 40% of patients presenting with possible DVT.8

However, the only way to definitively confirm the diagnosis of DVT is with further investigation in the form of imaging. Starting long-term anticoagulation without confirmatory imaging is poor clinical practice. Imaging is required in those low risk (“DVT unlikely”) patients who have a positive D-dimer, and in those at higher risk (“DVT likely”).


Duplex ultrasonography (USS) is now the most commonly used imaging modality in the UK for suspected DVT. The introduction of Doppler flow studies to show real-time imaging aids diagnosis even in the absence of direct visualisation of the clot. Occlusion of the vascular lumen is the major criterion for assessing clot presence but loss of the normal phasic signal from venous blood flow also suggests the presence of occluding clot.

Sensitivity ranges from 97% for proximal DVTs to 73% for distal DVTs.9 Clot more proximal to the inguinal ligament cannot usually be visualised on ultrasonography. Timing of the USS is important: if the USS is not available within 4 hours of being requested then the patient should be administered an interim 24-hour dose of parenteral anticoagulant until the scan is available.

Where persistent diagnostic doubt remains after duplex ultrasonography, or where the result cannot exclude a distal (calf vein) DVT (where there is debate over the benefit of intervention) further action will depend on the initial clinical assessment of risk. If the USS is indeterminate (rather than negative – see below) and it is important to make a firm diagnosis (e.g. in the early post-operative phase) then alternative imaging may be required (e.g. venography).

If the USS is negative and the pre-test risk stratification suggested that DVT was “likely” then current NICE Guidance is for the patient to undergo D-dimer testing at this stage. If the result of this is negative then DVT is excluded; if the D-dimer is positive, then the USS should be repeated approximately 1 week later.4 This pathway is summarised in the Diagnostic algorithm at the end of this section.

Learning bite

If the pre-test probability of DVT is “likely” and the USS is negative the patient should have a D-dimer to determine the need for a repeat delayed USS.