Polychromasia

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Polychromasia means a larger than normal number of immature red blood cells or reticulocytes (polychromatophilic cells) present in a blood sample that has been stained to examine it under the microscope. Polychromasia is often seen in regenerative anemia as the bone marrow is releasing more young red blood cells to compensate for those that have been lost, damaged or destroyed. Normally, immature red cells stay in the bone marrow for one or two days, but they are released early in cases of anemia in response to increased levels of erythropoietin (EPO).

Anemia is the usual cause of polychromasia, but it can also be due to damage to the bone marrow. Damage can be caused by scarring (fibrosis) or by tumors in the bone marrow itself or metastasis (secondary tumor). If polychromasia is seen, a reticulocyte count should be undertaken. If a high reticulocyte count is found, this generally indicates the presence of hemolysis (red cell destruction). It can sometimes take up to 96 hours for an increased reticulocyte count to be evident in a blood sample.

So, in dogs with AIHA/IMHA, it is not unusual to see polychromasia on the blood sample results & it is usually a response to anemia. Polychromasia is less pronounced in cats with anemia.