The importance of clinical probability assessment in interpreting a normal d-dimer in patients with suspected pulmonary embolism.

BACKGROUND: The d-dimer test is widely applied in the diagnostic workup of patients with suspected pulmonary embolism (PE). The objective of this study was to investigate how often the d-dimer test fails when clinical probability is not taken into account. METHODS: We used data collected in 1,722 consecutive patients with clinically suspected PE to analyze the 3-month venous thromboembolism (VTE) rate in all patients with a normal d-dimer concentration and separately for patients who have a normal d-dimer concentration with an unlikely or likely clinical probability for PE, as assessed by the Wells clinical decision rule. RESULTS: The 3-month VTE rate in all patients with a normal d-dimer concentration (n = 563) was 2.3% (95% confidence interval [CI], 1.4 to 3.9%). In the patients with an unlikely probability of PE (n = 477), VTE was confirmed in 1.1% of the patients with a normal d-dimer concentration (95% CI, 0.4 to 2.4%). In those patients with a likely clinical probability of PE (n = 86), VTE was confirmed in 9.3% of the patients with a normal d-dimer concentration (95% CI, 4.8 to 17.3%). The difference in VTE incidence between patients with unlikely and likely clinical probabilities of PE was significant (p < 0.001). CONCLUSIONS: Our findings indicate that it is of utmost importance to first examine the patient and assess the clinical probability, after which the d-dimer concentration can be taken into account, in order to prevent physicians from being influenced by a normal d-dimer test result when they evaluate the clinical probability of PE. Patients with a likely clinical probability should undergo further testing, regardless of the d-dimer test outcome.