Though less common than anemia, erythrocytosis, an increase in RBC mass is sometimes seen. The Hct (or PCV), Hgb, and usually RBC count are all increased.

Relative Erythrocytosis

Most often erythrocytosis is relative and due to dehydration. The total protein concentration will be elevated to the same degree unless there is a pre-existing reason for the protein to be low. In this case, the total protein may be within RI or decreased despite the dehydration. Restoration of normal fluid balance is required to properly assess the erythrogram and total protein.

Relative erythrocytosis can also occur with splenic contraction due to epinephrine release in an extremely excited animal, particularly the horse. Total protein is unaffected by splenic contraction.

Absolute Erythrocytosis

The increase in total red cell mass that occurs with absolute erythrocytosis is due to increased erythropoiesis in the bone marrow and can be classified as primary or secondary. Primary erythrocytosis is uncommon and is due to neoplasia of hemopoietic cells. If the neoplasm develops at the level of hemopoietic stem cells, then platelet and leukocyte numbers may also be elevated (variably called polycythemia vera or polycythemia rubra vera, in human medicine). However, this situation has not been recognized in veterinary medicine. When the neoplasm develops at the level of the erythroid precursor, only erythrocytes are increased in the peripheral blood and this is the condition recognized in veterinary patients, particularly cats and dogs. Erythropoietin concentrations are typically low or within the RI and arterial pO2 is within the RI since erythropoiesis is autonomous and not responding to a physiologic need for increased RBC mass.

With secondary erythrocytosis, erythropoietin concentrations may be increased for physiologic reasons because tissues are not being well-oxygenated. This is termed appropriate erythrocytosis and causes include cardiac or pulmonary disease leading to chronic hypoxia, and exposure to high altitudes over a long period of time. Alternatively, excess erythropoietin may be produced for nonphysiologic reasons unrelated to tissue oxygenation. This is termed inappropriate erythrocytosis and results from erythropoietin synthesis by various neoplasms and lesions, the most common being renal tumors.

Absolute vs relative erythrocytosis can usually be differentiated based on history, clinical findings, and additional laboratory findings. Absolute primary and secondary erythrocytosis (appropriate and inappropriate) can usually be differentiated based on ancillary tests (such as radiography, ultrasonography), arterial blood gases, and erythropoietin concentrations (Fig. 1.32).