Polycythemia (PG) corresponds to an increase in erythrocyte parameters on a blood test. A distinction is usually made between primary and secondary PG. The most common primary PG is Vaquez's disease, a hematological cancer. In Vaquez disease, an increase in hematocrit has been reported to be associated with a logarithmic increase in blood viscosity.

The main complications of primary PGs (especially in Vaquez disease) are thromboembolic complications. In contrast, thromboembolic complications are rarer in secondary PG. In Vaquez disease, a hematocrit ≤ 45% has been defined as the therapeutic goal for significantly reducing thromboembolic risk. However, this has not been established for secondary PGs. All in all, the definition of the 45% threshold is based solely on clinical studies with no obvious biological argument. What's more, simply lowering blood mass through cytoreduction alone does not appear to be sufficient to significantly reduce thromboembolic risk.

To investigator knowledge, there are no studies prospectively evaluating blood viscosity, its determinants and coagulation in different types of polycythemia. Nor are there any data on the direct effect on blood viscosity of the various treatments usually offered.

Inclusion Criteria:

• 18 years of age or older
• Followed for a diagnosis or suspicion of polycythemia (hematocrit greater than or equal to 49% in men and 48% in women), whatever the suspected etiology.
• Patient affiliated to a social security scheme or similar

Exclusion Criteria:

• Patient having undergone therapeutic bloodletting within 3 months prior to inclusion, or having initiated cytoreductive therapy prior to inclusion.
• Any disease or condition other than polycythemia, chronic or not, likely to induce a change in blood viscosity (at the investigator's discretion)
• Patient participating in another interventional research protocol that may interfere with the present protocol (at the investigator's discretion).
• Patient under guardianship, curatorship or safeguard of justice
• Person under psychiatric care
• Patient under legal protection.

Patients with polycythemia defined as hematocrit level greater than or equal to 49% in men and 48% in women, whatever the suspected etiology. Primitive and secondary polycythemia are eligible in this observational study. Patients should not have start any cytoreductive therapy prior inclusion.