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.