D-dimer Tests
The most important, simple test, available to emergency physicians to exclude a diagnosis of PE is the D-Dimer test. D-Dimer is a sensitive but not specific investigation which gives rise to its poor positive predictive value. In addition to VTE it is also elevated in the setting of infection, trauma, cancer and inflammatory diseases15. D-Dimer should therefore only be used to exclude PE in those patients in whom a negative result will allow confident exclusion of the diagnosis (PE Unlikely) group. D-Dimer should NOT be used to exclude PE in patients in the high clinical probability (PE likely) group, as a negative result has not been shown to exclude the diagnosis in these patients7. These patients should go on to have definitive imaging tests instead.
A positive test is used to inform further diagnostic approaches rather than treatment decisions.
Types of D-dimer test
There are two basic types of D-dimer assay:
• Qualitative bedside assays (e.g. SimpliRED, Simplify D-dimer)
• Quantitative ELISA or Latex laboratory based assays (e.g. Vidas or MDA)
Most qualitative tests can only reliably exclude pulmonary embolus in patients with a low clinical probability of the disease16. Quantitative tests, on the other hand, have been shown to be highly sensitive in excluding the diagnosis of PE in patients at either low or intermediate clinical probability17.
In the UK, NICE has suggested using D-Dimer only for patients classified as PE unlikely (using dichotomised Wells Score) and an ‘intermediate’ category of risk is not considered. NICE has evaluated evidence for both qualitative and quantitative assays and does not state a preference for either. However, it is important to be familiar with the type of assay available in your hospital, as different tests have varying sensitivities.
If carried out correctly, pulmonary embolus can be effectively excluded in those patients with a negative D-Dimer (A negative D-Dimer result will reduce the risk of PE to less than 1% if the pre-test probability is less than 20%) and no further imaging is required. It is important to ensure that an alternative diagnosis is sought to explain the patient’s symptoms.
Learning Bite
It is important to know which type of D-Dimer assay is used in your hospital.
Learning Bite
In low risk patients with negative investigation for PE it is important to ensure that an alternative diagnosis is sought to explain their symptoms.
Application of D-dimer testing
The results of D-Dimer testing should only be considered in the context of the clinical probability of PE derived from one of the clinical risk tools previously described.
Depending on the clinical situation and the results of these more simple tests, more complex tests may be required.
“A diagnostic strategy for patients attending the ED with symptoms suggestive of pulmonary embolus has been published by NICE14 as part of wider guidance on the diagnosis and management of VTE. The most up to date version was published by NICE in 2020.”
