Assays of the d-dimer fibrin degradation product in the circulation have become ‘indispensible’ in the evaluation of patients with suspected venous thromboembolism (VTE: deep venous thrombosis and/or pulmonary embolism). But how useful are they, and are they used wisely?
Like many diagnostic aids, the utility of d-dimer assays is influenced by the clinical setting (to be more precise, the a priori or ‘pre-test’ probability that the disease being tested for is present). Unfortunately, this is a test with a very low specificity, albeit a high sensitivity [1], and a significant problem for the diagnostician using this test is that d-dimers behave like ‘acute phase’ reactants and can be elevated in a wide range of situations associated with an acute phase response, such as inflammation, infection and malignancy. The trouble is that many of these feature in the differential diagnosis of presentations suggesting venous thromboembolism (e,g. pleuritic chest pain:pleurisy, shortness of breath/hypoxia:pneumonia, painful swollen leg:cellulitis). And, to cap it all, patients with malignant disease are at high risk of VTE, but may have elevated d-dimers for other reasons [2].
In practice, the d-dimer test is only useful if the result is not elevated and the clinical probability of VTE is low, when it adds confidence to a conclusion that VTE is not present and that further investigation with scanning is unnecessary. Diagnostic dilemmas arise commonly, however, when the clinical probability of VTE is low but the d-dimer result is elevated. An elevated CRP in this situation indicates an acute phase response and renders the d-dimer result virtually worthless. An elevated d-dimer with a normal CRP may be more significant, but I don’t think the utility of the d-dimer assay in that situation has yet been adequately studied. In general, it is best not to order a d-dimer test if there is a high clinical probability that it will be elevated anyway.
Similarly, if the clinical suspicion of VTE is anything but very low then don’t bother with a d-dimer assay, just get on and do the appropriate scans.
