Take into consideration alternative diagnoses in patients with:

Relative values of different outcomesThe number of DVT cases missed (false negatives) and the number of false positives (when treatment may be started incorrectly) are the most important outcomes. It is also important that patients are reassured that they do not have DVT, but have information about when to come back if they have more signs and/or symptoms of a possible DVT.
Trade off between clinical benefits and harmsThe benefit to informing the patient that they are unlikely to have a DVT is that other diagnosis can then be considered and that no further investigation into a DVT is necessary. If no further tests are pursued, there is a small possibility that a DVT may be missed, but this possibility is minimised with the diagnostic strategy recommended: The risk of DVT for any of the above groups is very low; they either had a low pre-test probability and a negative result from a sensitive test, or had a higher pre-test probability but tested negative with two different high sensitivity tests. It is not beneficial to subject these patients to further tests because the probability of having missed a DVT is very low.
In the unlikely event that these tests missed a DVT, patients need to know about the signs and symptoms of DVT and when or where to seek further help or advice. Therefore, this information should be given to all patients who presented with a suspected DVT.
Economic considerationsBased on a decision model comparing different sequences of tests, ruling out a diagnosis of DVT is cost-effective when a patient has:
Quality of evidenceNo specific clinical evidence review was conducted for this area. This recommendation is supported by GDG consensus and information by economic evidence.
The economic evidence has potentially serious limitations and partial applicability.
Other considerationsD-dimer is a sensitive test that is useful in excluding DVT in combination with a DVT Wells score which stratified patients into the appropriate pre-test probability categories. The evidence review suggests that the risk of patients actually having DVT is low if their DVT Wells score is “unlikely” and a D-dimer test is negative test, and this strategy can potentially exclude a large proportion of patients presenting with suspected DVT.
For patients with a ”likely” DVT Wells score, but a negative ultrasound scan, a negative D-dimer test helps to further eliminate the possibility that the patient has a DVT.
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.

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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