The role of serum erythropoietin levels in diagnosis and classification of erythrocytosis/polycythemia.

I read with great interest the recent paper of Gordeuk et al. on congenital polycythemias/erythrocytoses, in which a diagnostic algorithm based on serum erythropoietin (Epo) concentration in non-phlebotomized patients is presented. As a first step the authors propose classification into primary erythrocytosis (increased red cell mass due to intrinsically abnormal bone marrow erythropoiesis) versus secondary erythrocytosis (excessive erythroid proliferation in response to pathological events outside the progenitor cells) according to whether the serum Epo level is below or above 5 IU/L. Primary erythrocytoses are further distinguished into primary familial and congenital polycythemia (PFCP) and polycythemia vera (PV), the latter being acquired but occasionally occurring in a familial or juvenile context, whereas secondary congenital polycythemias can be subdivided into disorders of hypoxia sensing, conditions of increased affinity of hemoglobin for oxygen (hemoglobin mutants, 2,3-bisphosphoglycerate deficiency, methemoglobinemia) and congenital cyanotic heart or lung disease. The use of a unique clear-cut serum Epo level as classification criterion for primary versus secondary erythrocytosis, however, does not correspond to observations in a number of studies evaluating the discriminating value of serum Epo levels in polycythemic patients. Low serum Epo values have high specificity and moderate sensitivity for diagnosis of primary polycythemia compared with other erythrocytoses (Table 1).