The purpose of this study is to develop a way to predict which patients diagnosed with idiopathic blood clots can safely stop warfarin therapy after six months. We will use patient characteristics, blood test results and imaging test results to identify those patients who have the lowest risk of developing a new blood clot after warfarin is stopped.

The risk of recurrent VTE in patients with idiopathic VTE subsequent to three to six months of oral anticoagulant therapy remains high (5-27% per year). The risk of recurrent VTE, however, is not likely high enough to justify indefinite anticoagulation in all patients with a first idiopathic VTE due to the rate of major bleeding with oral anticoagulants (2-6% per year), the inconvenience and cost of oral anticoagulant therapy, monitoring of oral anticoagulant therapy (e.g. prescription costs, time off work to go for lab tests, parking etc) and the lifestyle limitations of oral anticoagulant therapy (avoidance of certain physical activities, dietary restrictions, avoidance of pregnancy). Further, as some have argued, perhaps all that anticoagulant therapy achieves is to delay recurrent VTE and, as a recent editorial suggested, a tailored approached is required to determine sub-groups who require lifelong anticoagulation. A means to stratify patients with idiopathic VTE to identify a group at low risk of recurrent VTE who could safely discontinue oral anticoagulants subsequent to six months of therapy would be a significant advance in the care of these patients.

Inclusion Criteria:

1. Objectively proven (as previously described (8)) proximal idiopathic deep vein thrombosis or pulmonary embolism. Idiopathic will be defined as VTE occurring in the absence of fracture, plaster cast, immobilization greater than 3 days or a general anesthetic in the last three months prior to VTE diagnosis; a known deficiency of antithrombin, protein C or protein S; and malignancy in the last five years
2. Patients treated initially with a minimum of five days of heparin or low molecular weight heparin and oral anticoagulants with a target intensity of 2.0 - 3.0 with no recurrence in the subsequent six months.
3. Patients currently on oral anticoagulants

Exclusion Criteria:

1. Recurrent idiopathic VTE (i.e. ≥ 2 previous idiopathic VTE). Previous secondary VTE is not an exclusion criterion;
2. Age <18;
3. Known deficiency of proteins S, protein C or antithrombin;
4. Known, persistently positive anticardiolipin antibodies (titers > 30U/ml);
5. Known, persistently positive lupus anticoagulant;
6. Combined thrombophilic defects (e.g. homozygous for FVL or PGM, or compound heterozygous for FVL and PGM);
7. Refusal of informed consent.

patients diagnosed with idiopathic blood clots