Blood p50 evaluation enhances diagnostic definition of isolated erythrocytosis

Background. High oxygen-affinity haemoglobin variants and 2,3-diphosphoglycerate (2,3-DPG) deficiency are inherited diseases generating low tissue oxygen tension and erythropoietin-driven erythrocytosis, that characterizes the clinical phenotype of patients. Level of blood p50 (the oxygen tension at which haemoglobin is 50% saturated) is used to recognize these conditions. Objectives. To define the clinical utility of blood p50 measurement in the diagnosis of isolated erythrocytosis. Subjects and design. Venous blood p50 measurement was included in the diagnostic work-up of 102 consecutive patients with isolated erythrocytosis besides blood cell count, arterial oxygen saturation, serum erythropoietin measurement and screening for JAK2 mutations. Setting. Haematological Outpatient Section at University Hospital. Results. Seven patients had relative erythrocytosis. Among 95 patients with absolute erythrocytosis, 4 (4.2%) had decreased p50 level. The extended study of family members revealed a familial inheritance. Two families had haemoglobin variants already described as Haemoglobin Malmö and Haemoglobin San Diego. In one family, the proband had a new high oxygen-affinity haemoglobin variant (Haemoglobin Safi) resulting from the transversion C→A at codon 81 of the α2-globin gene. In the last family, a deficiency of 2,3-DPG was found. Within the 91 patients with normal p50 values, 46 (51%) had secondary erythrocytosis, 13 (14%) polycythemia vera and 32 (35%) idiopathic erythrocytosis. Conclusions. This study suggests that the investigation of blood p50 level may be useful to define diagnosis in patients with isolated erythrocytosis.