D-dimer Testing Tailored to Clinical Pretest Probability in Suspected Pulmonary Embolism

Inclusion Criteria: * Patients (either outpatients or inpatients) with clinically suspected Pulmonary Embolism. Exclusion Criteria: 1. Age less than 18 years. 2. Treated with full-dose anticoagulation for ≥ 24 hours before D-dimer was measured. 3. Major surgery (general or spinal anesthesia) in the past 21 days. 4. Result of the D-dimer assay that will be used to manage the patient in the study is known before Clinical Pretest Probability was done. 5. Computed Tomography Pulmonary Angiogram or Ventilation Perfusion Scan was performed: 1. before Clinical Pretest Probability was documented, or 2. in a patient with Low Clinical Pretest Probability and a D-dimer level \<1,000 ug/L (or equivalent), or 3. in a patient with Moderate Clinical Pretest Probability and a D-dimer level \<500 ug/L (or equivalent). 6. Computed Tomography of the chest with contrast will be performed for another reason (e.g. to assess for malignant disease or aortic dissection), and would be performed even if Pulmonary Embolism is excluded by Clinical Pretest Probability and D-dimer testing. 7. Ongoing need for anticoagulant therapy. 8. Life expectancy less than 3 months. 9. Geographic inaccessibility which precludes follow-up. 10. Known pregnancy. 11. Inability to provide informed consent.