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 Evaluation of erythrocyte numbers can begin with a determination of the packed cell volume (PCV) or hematocrit (HCT), which measure the percentage of the volume of whole blood that the red blood cells (RBCs) occupy.

 Erythrocyte numbers are 6.9–10.7 x 1012/l, which represent a normal PCV of 35–45% and a normal hemoglobin of 12–15 g/dl.

 A stained blood smear can be used to evaluate RBC morphology and the presence of infectious organisms on the RBCs.

 Increased RBC numbers (erythrocytosis) is most commonly associated with hemoconcentration – this is called a relative erythrocytosis as there is no increase in RBC mass.Splenic contraction can cause a transient increase in circulating RBC mass.

 In rare cases, there is an increase in numbers due to an increase in RBC production, absolute erythrocytosis.Absolute erythrocytosis is further categorized as primary, as in polycythemia vera, and secondary, as results from chronic hypoxemia.

 Decreased RBC numbers (anemia) can be caused by increased removal from the circulation by blood loss or destruction (hemolysis); or by decreased production by the bone marrow.Intravascular hemolysis is accompanied by a decrease in hemoglobin concentration; however, extravascular hemolysis is harder to confirm.Decreased production with iron deficiency secondary to chronic inflammation is a common cause of nonregenerative anemia in the horse, and is characterized by microcytic hypochromic RBCs and decreased RBC indices.Red cell distribution width (RDW) – a measure of RBC variation in size and volume ‐ is used to detect RBC regeneration and is interpreted in conjunction with other values on the complete blood count (CBC).