Myeloproliferative neoplasms (MPN) such as Polycythemia Vera (PV) and, Essential Thrombocythaemia (ET) are rare clonal myeloid neoplasms associated with an increased risk of both venous and arterial thrombosis. Thrombotic complications are the main determinant of morbidity and in a less extend mortality.

Routine haemostasis analysis (TP, aPTT) are usually normal and are useless to demonstrate a hypercoagulable state. However, previous evidence suggests that global coagulation tests such as thrombin generation or thromboelastometry are able to detect signs of procoagulant imbalance in MPN. Similarly, current data seems to demonstrate that fibrin clot properties (clot permeability, turbidimetry, clot lysis time) properties is altered suggesting an hypercoagulable state.

Goals of PV and ET treatments are to control blood count to reduce the risk of thrombotic events. Moreover, new drugs such as Janus Kinase Inhibitors (JAKi) were recently licensed for PV and are under investigations on clinical trial for ET. It is currently unknown if treatments that were used for ET and PV, and especially JAKi are able to modify the hypercoagulable state that is observed in those diseases, and if there is difference between drugs.

To evaluate impact of MPN treatment on prothrombotic haemostatic profile, we propose to evaluate global coagulation and fibrin clot properties in MPN, depending on the treatment.

Inclusion Criteria:

• All men and women, older than 18 years, with a diagnosis of PV or ET (primary or secondary) according to the 2008 World Health Organization (WHO) classification.

Exclusion Criteria:

• Lack of participant's consent;
• Concomitant treatment with anticoagulant drugs (anti-vitamin K, heparin or direct oral anticoagulant drugs);
• Active cancer other than non-melanoma skin cancer (defined as cancer diagnosis <5 years or treatment <2 years);
• Recent infection (<30d);
• Recent surgery (<30d);
• Recent hospitalization (<30d);
• Recent thromboembolic or cardiovascular event (<3m).

Screening for participation for MPN patients will occur at the outpatient clinic of the division of Haematology at the Geneva University Hospitals (Rue Gabrielle-Perret-Gentil 4, 1205 Geneva, Switzerland) and at the outpatient clinic of the division of Haematology at Guy's Hospitals (Great Maze Pond, London SE1 9RT, United Kingdom).