Recommendations
3.

Offer patients in whom DVT is suspected and with a likely two-level DVT Wells score either:

Repeat the proximal leg vein ultrasound scan 6–8 days later for all patients with a positive D-dimer test and a negative proximal leg vein ultrasound scan.

Relative values of different outcomesThe GDG considered the avoidance of undiagnosed and untreated DVT to be the most important outcome, followed by concerns about the number of additional diagnostic tests (which are non invasive, with few side effects) that patients receive. This recommendation is intended to follow up patients with the appropriate tests after the pre-probability testing with a DVT Wells score. The ability to correctly confirm and initiate treatment for patients with DVT while sending patients who do dot have DVT home without further imaging or treatments are considered the most important issues.

Both sensitivity and specificity are important outcomes. In this situation, D- dimer was considered in the context of ruling out DVT.
Trade off between clinical benefits and harmsAs sensitivity increases (less patients with DVT missed), the proportion of patients with a false positive test may increase (more patients sent for unnecessary further investigations and treatments and this is an important strain on the NHS resources).

D-dimer
D-dimer tests have relatively high sensitivity but low specificity (false positive results common). When the sensitivity of a d-dimer test increase, its specificity decreases. To be useful in the diagnosis of DVT, a D-dimer test has high sensitivity and high negative value - fewer people with DVT will be missed. Therefore, a negative D-dimer may be useful in excluding DVT but a positive D- dimer is of no diagnostic value, it merely mandates further testing. Whilst a negative D-dimer test is good enough to exclude the diagnosis of DVT in a patient with an “unlikely” pre-test clinical probability it is not good enough in those with a “likely” pre-test probability.
Proximal leg vein ultrasound scan
Proximal leg vein ultrasound scans are used as confirmatory tests in this pathway. Therefore both sensitivity and specificity are important, in order to ensure all DVTs are detected, and patients without DVT are not given heparin. The GDG had recommended proximal leg vein ultrasound scans as the clinical importance of picking up extra calf vein blood clots by scanning the whole leg is uncertain. Moreover, the evidence review suggested that ultrasound scan of calf veins are not very sensitive in picking up calf vein DVT. A repeat proximal leg vein scan is recommended to ensure that any clots propagating to the proximal veins are not missed.

It is important to follow the sequence recommended to minimise the unnecessary use of ultrasound scans so that patients who need these scans can access them as soon as possible. Patient can be at risk of deterioration or at risk of a PE If a quick confirmation scan is not available. That is why anticoagulants are recommended if there is a delay in getting access to a scan.
Economic considerationsBased on a decision model comparing different sequences of tests, after risk stratification with a DVT Wells score, offering an ultrasound scan is cost- effective in the high risk group or after a positive D-dimer test. According to this model, offering a D-dimer test to patients who had a negative ultrasound scan is cost-effective. The cost of performing a D-dimer test is relatively low (between £19 and £20).

Above-knee ultrasound with a repeat if negative is more cost-effective than a single above-knee or full-leg ultrasound scan.

The four-hour limit to the ultrasound scan was not based on economic evidence but on safety considerations.

The model was conducted using a three-level DVT Wells score but based on other considerations on implementation the GDG decided to recommend a two-level DVT Wells score.
Quality of evidenceD-dimer
The majority of the evidence base comes from a large meta-analysis which pooled 97 diagnostic studies. The pooled sensitivity is 90%, indicating that 90% of patients with DVT will be correctly picked up. However, the main limitation of this evidence is this is a form of “average” sensitivity of all D-dimer tests. The actual sensitivity of tests varies between about 80% to more than 90%, depending on the specific type of technology used in the tests.
Proximal leg vein ultrasound scan
The quality of evidence ranged from very low to moderate for the various ultrasound strategies reviewed. These studies showed that ultrasound scans have high specificities, which makes them effective in confirming the presence of DVT. However, the sensitivity of the tests can vary a little between studies and average around 90%.

The economic evidence has potentially serious limitations and partial applicability.
Other considerationsProximal ultrasound will be used to confirm whether patients have DVT if they presented with DVT symptoms and accessed as “likely” risk of DVT using a two- level DVT Wells score.

The GDG considered at length the implications of implementation and whether this affects current practice. The following factors were discussed and considered by GDG members:
  • It is important to diagnose and confirm DVT quickly. Treatment with LMWH exposes patients to side effects and is expensive (cost of drug and district nurse time). It is important not to put patients needlessly on LMWH.
  • It is necessary to find a safe and cost-effective strategy to identify which patients can be sent home safely (through DVT Wells score and D-dimer), and reduce the number of people referred for an ultrasound scan.
  • Access to ultrasound scan can be a problem, especially at weekends and outside normal working hours. Delays in accessing ultrasound scans are a potential problem and these delays need to be addressed and avoided. In situations where delay in access is unavoidable, strategies are required to ensure that patients are treated in the interim.
  • Therefore, while it is recognised that access to ultrasound scans can be a limitation, it was also agreed that this should not be a reason on its own to prevent recommending what is required in the best interest of patients, especially when this is a very cost effective strategy. The GDG had considered that since patients assessed as having a high risk of DVT will not be sent home even if a D-dimer is negative, it is best to prioritise sending this group of patients to ultrasound scans so that a diagnosis can be confirmed and treatment initiated promptly.
  • In patients with a “likely” DVT Wells score, patients with a positive ultrasound scan have DVT confirmed and need to be treated immediately, while patients with a negative ultrasound scan are offered a D-dimer to double check that there is a low risk of DVT before being sent home.
The GDG also discussed that ultrasound techniques have important limitations in visualising iliac vein thrombosis. The current clinical understanding is this technique may not be effective if the relatively unusual situation of isolated iliac vein thrombosis is suspected. If this is suspected (for example, from changes in blood flow in the femoral vein), the usual practice is to investigate with other imaging methods such as CT or MR venography.

The GDG prioritised this recommendation as a key priority for implementation. They considered that it has a high impact on outcomes that are important to patients, a high impact on reducing variation in care and outcomes, leads to a more efficient use of NHS resources, promotes patient choice, promotes equalities and means patients reach critical points in the care pathway more quickly.

From: 5, Diagnosis of deep vein thrombosis

Cover of Venous Thromboembolic Diseases
Venous Thromboembolic Diseases: The Management of Venous Thromboembolic Diseases and the Role of Thrombophilia Testing [Internet].
NICE Clinical Guidelines, No. 144.
National Clinical Guideline Centre (UK).
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