This observational study will use a new way to test for deep vein thrombosis (DVT) of the leg. A DVT is a blood clot in the leg and is a medical problem that can cause swelling, pain, and redness. If the blood clot is not treated, it can cause more serious, long-term effects, and occasionally lead to death.

The main questions the study aims to answer are:

1. How safe is our new blood clot testing method?
2. How efficient is our new blood clot testing method?

The study will be run in the emergency department and urgent care centre in Kingston, Ontario. Patients who are tested for a DVT in the leg can be included in the study. Researchers follow the patient through chart review to make sure the new system is safe and efficient.

Inclusion Criteria:

- emergency physician decides to test for DVT of the lower limb.

Exclusion Criteria:

• <18 years of age;
• tested for DVT without using the ToDay algorithm at the index visit;
• D-dimer level is known before ToDay item assessment;
• a new indication for anticoagulation at the emergency department assessment (other than DVT) in a patient not previously prescribed an anticoagulant;
• a history of prior lower limb DVT, where there is no access to baseline ultrasound imaging;
• suspected of having a pulmonary embolism;
• has previously participated in this study;
• has previously opted out of all research at hospital site;
• left the emergency department or urgent care centre against medical advice;
• D-dimer was ordered prior to initial physician assessment;
• imaging for DVT or pulmonary embolism within the previous 30 days;
• transferred from another hospital organization.

Study participants will be enrolled after a visit to the Kingston Health Sciences emergency department and urgent care centre in Kingston, Ontario.