 - In JAK2V617F positive myeloproliferative neoplasms MPN with hematocrit and hemoglobin levels under the PV cut-off in order to discriminate between masked PV and essential thrombocythemia CO-Rebreathing is a fast, non invasive alternative method for RCM evaluation. It is based on the high affinity of carbon monoxide CO for hemoglobin. The decrease in the HbCO percentage after CO inhalation is dependent on the total hemoglobin mass from which RCM can be obtained. The investigators propose a tri-centric study evaluating CO-Rebreathing as an alternative tool to measure RCM in the diagnosis of polycythemia in the two categories of patients described above. 