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4.4 D-dimer testing
D-dimer levels are elevated in plasma in the presence of acute thrombosis because of simultaneous activation of coagulation and fibrinolysis. The negative predictive value of D-dimer testing is high, and a normal D-dimer level renders acute PE or DVT unlikely. On the other hand, the positive predictive value of elevated D-dimer levels is low and D-dimer testing is not useful for confirmation of PE. D-dimer is also more frequently elevated in patients with cancer, in hospitalized patients, in severe infection or inflammatory disease, and during pregnancy. Accordingly, the number of patients in whom D-dimer must be measured to exclude one PE (number needed to test) rises from 3 in the general population of an emergency department to >_10 in the specific situations listed above.
As a number of D-dimer assays are available, clinicians should
become aware of the diagnostic performance of the test used in their own hospital. The quantitative enzyme-linked immunosorbent assay (ELISA) or ELISA-derived assays have a diagnostic sensitivity of >_95%, and can be used to exclude PE in patients with either low or intermediate pre-test probability. In the emergency department, a negative ELISA D-dimer can, in combination with clinical probability, exclude the disease without further testing in 30% of patients with suspected PE. Outcome studies have shown that the 3 month thrombo embolic risk was <1% in patients with low or intermediate clinical probability who were left untreated on the basis of a negative test result.
4.4.1 Age-adjusted D-dimer cut-offs
The specificity of D-dimer in suspected PE decreases steadily with age
to 10% in patients >80 years of age. The use of age-adjusted cutoffs may improve the performance of D-dimer testing in the elderly. A multinational prospective management study evaluated a previously validated age-adjusted cut-off (age x 10 µg/L, for patients aged >50 years) in a cohort of 3346 patients. Patients with a normal ageadjusted D-dimer value did not undergo CTPA; they were left untreated and followed for a 3 month period. Among the 766 patients who were >_75 years of age, 673 had a non-high clinical probability.
Use of the age-adjusted (instead of the ‘standard’ 500 µg/L) D-dimer cut-off increased the number of patients in whom PE could be excluded from 6.4 to 30%, without additional false-negative findings.