Abstract BACKGROUND:The diagnostic workup of pulmonary embolism (PE) may take several hours. The utility of anticoagulant treatment while awaiting the results of diagnostic tests has not been assessed. OBJECTIVE:To compare the risks and benefits of twice-daily low-molecular-weight heparin versus no treatment in patients with suspected PE. METHODS:We developed a decision tree with the following outcomes: mortality related to untreated and treated PE, mortality due to major hemorrhage, and intracerebral bleeding. The timeframe extended from the suspicion of PE to its confirmation or exclusion. Most probabilities were derived from data of the RIETE registry. We estimated the incidence of bleeding by categories of clinical prediction rules of PE from a recent diagnostic management study of PE. Uncertainty was assessed through one-way and probabilistic sensitivity analyses. RESULTS:The model favored preemptive anticoagulation if the diagnostic delay was >6.3h, >2.3h and >0.3h (Revised Geneva low, intermediate and high probability) and >8.1h and >1.7h (Wells unlikely and likely). With a diagnostic delay of 6h, the absolute mortality reduction with anticoagulation was 0%, 0.02% and 0.1% for low, intermediate and high clinical probability. In one-way sensitivity analyses, the mortality of untreated PE was the most critical variable. Probabilistic analyses reinforced the superiority of anticoagulation in intermediate and high probability patients, and suggested that low probability patients might not benefit from treatment for diagnostic delays <6-8h. CONCLUSIONS:Our model suggests that patients with intermediate and high/likely probabilities of PE benefit from preemptive anticoagulation. With a low probability, the decision to treat could rely on the expected diagnostic delay.