This is a cohort study that will assess a new diagnostic management strategy for suspected Deep Vein Thrombosis in outpatients.The new diagnostic strategy is designed to reduce the use of ultrasound testing on the day of presentation, and reduce repeat ultrasound testing a week after an initial normal test.

This is a prospective, multicentre, cohort study that will assess a new diagnostic management strategy for suspected Deep Vein Thrombosis in outpatients. The new diagnostic strategy is designed to reduce the use of ultrasound testing on the day of presentation, and reduce repeat ultrasound testing a week after an initial normal test. Less ultrasound testing will be performed because: i) more patients will have deep vein thrombosis excluded by combinations of Clinical Pretest Probability and D-dimer results on the day of presentation; and, ii) in those who still need an ultrasound, a repeat ultrasound a week after a normal result will only be performed if the D-dimer result is markedly abnormal at initial presentation. The safety of this management strategy will be established by demonstrating a very low rate of proximal Deep Vein Thrombosis or Pulmonary Embolism during 90 days follow-up in patients who had anticoagulant therapy withheld in response to negative diagnostic testing. Diagnostic test utilization will be assessed. All clinical outcomes will be adjudicated by a central independent adjudication committee that will be blind to initial D-dimer measurements and patient management.

Inclusion Criteria:

1) Consecutive outpatients (i.e. non-hospitalized patients) with clinically suspected Deep Vein Thrombosis.

Exclusion Criteria:

1. Age less than 18 years.
2. Treated with full-dose anticoagulation for ≥ 24 hours before D-dimer was measured.
3. D-dimer level known before Clinical Pretest Probability documented.
4. Ultrasound of the leg performed before Clinical Pretest Probability was documented
5. Ultrasound was or will be performed in a patient with low Clinical Pretest Probability and a d-dimer <1000(or equivalent), or with Moderate Clinical Pretest Probability and a D-dimer <500 (or equivalent) (e.g. suspected bleeding).
6. Ongoing need for anticoagulant therapy.
7. Suspected Pulmonary Embolism.
8. Superficial venous thrombosis that requires, or may require, anticoagulant therapy.
9. Life expectancy less than 3 months.
10. Previously confirmed episode of Deep Vein Thrombosis.
11. Geographic inaccessibility which precludes follow-up.
12. Known pregnancy.

Male and females outpatients with a suspected Deep Vein Thrombosis