What is erythrocytosis?

By William Aird

An increase in the number of red blood cells (RBCs) relative to the plasma volume. It is manifested by a persistent increase in the venous hematocrit:

  • Relative erythrocytosis – elevated hematocrit associated with normal red cell mass, resulting from contracted plasma volume most often due to dehydration.
  • Absolute erythrocytosis:
    • An increased red cell mass above 125% of the predicted value for the body mass of the patient.
    • Red cell mass is rarely measured; hemoglobin and/or hematocrit typically used as surrogates for red cell mass.