Post-thrombotic syndrome (PTS) is the most common chronic complication of deep vein thrombosis (DVT), with major consequences for patient quality of life and cost of management. Identifying patients at high risk of developing PTS could be useful for its prevention and may lead to more appropriate therapeutic strategies to reduce its incidence and severity.

Prognostic tools for predicting risk are very useful for choosing the optimum treatment and improving patient management and are a preliminary step before developing predictive models useful for determining sensitivity to treatment. At present, although several prognostic markers and models have been proposed, it is still difficult to predict who will develop a PTS or a moderate to severe PTS. The development of PTS is multifactorial and depends largely on the extent and severity of venous obstruction which supports the theory of thrombosis burden (DVT-Burden) as a potential prognostic marker for PTS. It therefore seems important to study the association between thrombosis burden and the occurrence of PTS.

The Venous Volumetric Index or VVI (Ouriel 1999) will be used for quantifying DVT-Burden. The VVI was constructed by calculating the volume from the diameter and length of 14 venous segments from the calf veins to the inferior vena cava. The VVI has been validated for its ability to discriminate between symptomatic and asymptomatic DVT and has shown superior performance to other methods for quantifying DVT.

This study aim to assess the performance of baseline DVT-burden estimated by the VVI score on ultrasound for predicting the occurrence and the severity of PTS as assessed by the Villalta scale at 6 months.

This is a multicenter prospective cohort study aiming at assessing baseline DVT-Burden and other prognostic factors for predicting the occurrence and the severity of PTS.

Patients diagnosed with a first episode of DVT of the lower limbs are recruited in offices and departments of vascular medicine. They will be informed of the study by their physician. If patients agree to take part and meet the eligibility criteria, they will be included consecutively in the study after signing an informed consent form.

The study will include follow-up visits at one week (D7±2, for patients participating in the biological research only), 1 month (D30±5), 3 months (D90±5) and 6 months (D180±5).

At each visit, the following examinations will be carried out:

• Assessment of symptoms and clinical signs to evaluate the Villalta score.
• Venous ultrasound evaluation of the lower limbs by colour Doppler ultrasound (CDUS). Data collected will be useful to calculate the VVI score planned at the study analysis phase.

At the D0, D7, D30 and D90 visits, blood samples will be taken for research purposes to assess factors of inflammation, coagulation and fibrinolysis.

At the D90 and D180 visits, the patient will also be asked to complete the VEINES-QOL and SF-36 quality of life questionnaires.

The patient's participation in the research will end at the end of the D180 visit.

Inclusion Criteria:

1. Age ≥ 18 years
2. Patients with symptomatic acute deep venous thrombosis of the lower limbs confirmed by ultrasound on the criteria of venous incompressibility and direct image of the thrombus (patients with stable pulmonary embolism associated with a symptomatic deep venous thrombosis can be included)
3. Affiliates or beneficiaries of a social security scheme.

Exclusion Criteria:

1. Pregnant women, women in labour or breastfeeding mothers.
2. Pulmonary embolism haemodynamically unstable defined by systolic blood pressure < 90 mmHg or a drop in systolic blood pressure of at least 40 mmHg during at least 15 min, shock or cardiac arrest.
3. Asymptomatic venous thrombosis.
4. Symptomatic venous thrombosis of both lower limbs (patients with bilateral deep venous thrombosis but symptomatic on one side only can be included).
5. History of ipsilateral or contralateral venous thrombosis of the lower limb.
6. Fracture or orthopedic surgery of the lower limbs in the last 3 months.
7. Dependance caused by age or secondary to a chronic affection, going from the inability to stand up or walk alone without help to bed or armchair rest over.
8. Prophylactic or therapeutic anticoagulant treatment > 5 days.
9. Expected duration of anticoagulant treatment < 3 months (all patients must have a minimum treatment of 3 months).
10. Patient estimated at high risk of bleeding according to investigator clinical judgment (renal, platelet, digestive, hepatic, neurological... origin) .
11. Indication for interruption of the inferior vena cava or venous recanalisation (endovascular, thrombolysis or surgery).
12. Refusal or inability to give written informed consent to participate in the study.
13. Life expectancy < 6 months.
14. Patients under legal protection (guardianship, curatorship, etc.) or safeguard of justice.
15. Patients taking part in a research project for venous thromboembolism that can interfere with the conduct of DVT-BURDEN research.