The purpose of this study is to compare two diagnostic interventions to improve the way D-dimer blood testing (MDA D-dimer) is used to diagnose first time symptomatic deep vein thrombosis.

• Limiting use of D-dimer testing to outpatients with a Low or Moderate clinical pretest probability (C-PTP)for deep vein thrombosis AND using a D-dimer level of < 1.0 µg FEU/mL to exclude deep vein thrombosis in those with a Low C-PTP, and a D-dimer level of < 0.5 µg FEU/mL to exclude deep vein thrombosis in those with a Moderate C-PTP, is as safe and a more efficient way to diagnose DVT than:
• Performing D-dimer testing in all patients with suspected deep vein thrombosis with use of a single D-dimer value of < 0.5 µg FEU/mL to exclude thrombosis (current practice).

All randomized patients, including those who are treated for deep vein thrombosis after initial testing, will be followed for a period of 3 months to monitor for signs and symptoms suggestive of deep vein thrombosis, pulmonary embolism, bleeds and death.

Inclusion Criteria:

• Age 18 and older
• Presenting with symptoms compatible with clinically suspected deep vein thrombosis

Exclusion Criteria:

• Treatment with full dose anticoagulation for 24 hours or more.
• Other test for deep vein thrombosis already performed.
• Ongoing need for therapeutic anticoagulant therapy.
• Life expectancy less than 3 months.
• Absence of acute symptoms within 7 days of presentation.
• Presenting with symptoms of pulmonary embolism.
• Previous confirmed episode of deep vein thrombosis or pulmonary embolism.
• Current pregnancy.
• Geographic inaccessibility which precludes follow-up.