Clinical usefulness of D-dimer depending on clinical probability and cutoff value in outpatients with suspected pulmonary embolism.
We evaluated whether a highly sensitive D-dimer test is clinically useful and safe for ruling out pulmonary embolism (PE) in patients with a high clinical probability and whether adopting different cutoff values according to the clinical probability category might increase the proportion of patients in whom PE is ruled out. We retrospectively analyzed the databases of 2 outcome studies on the diagnosis of PE with a 3-month follow-up that included 1409 patients. We evaluated the usefulness of D-dimer testing by calculating the number needed to test to rule out one PE, and its safety by measuring the 3-month thromboembolic risk in patients not treated by anticoagulant agents based on a normal D-dimer level. The sensitivity of D-dimer was 100% in all clinical probability categories, but the number needed to test increased with increasing clinical probability of PE. The 95% confidence interval (0%-23%) of the 3-month thromboembolic risk (0%) among 13 of 121 patients with a normal D-dimer level and a high clinical probability of PE was wide. Increasing the cutoff value to 700 mug/L in patients with a low clinical probability would rule out PE in an additional 5% of the entire patient cohort at the expense of a lower sensitivity (93% [95% confidence interval, 83%-97%]). The safety of D-dimer testing in patients with a high clinical probability of PE is not established, and testing results are rarely negative in such patients. Increasing the enzyme-linked immunosorbent assay D-dimer cutoff value only marginally increased the test's usefulness.