Morphology: immature red blood cells that stain dark purple on Romanowsky stains (Wright’s, Diff-Quick) due to retained cytoplasmic RNA. Larger than normal red cells.
Polychromatophils versus reticulocytes: both are immature red blood cells containing RNA. Reticulocytes stain blue with new-methylene blue. Polychromatophils stain purple with Romanowsky stains. Aggregate reticulocytes stain purple on Romanowsky stains so aggregate reticulocytes are polychromatophils. Punctate reticulocytes cannot be distinguished from normal red blood cells on Romanowsky stains.
Clinical relevance: increased polychromatophils (polychromasia) indicates that bone marrow is responding to anemia. Anemia is considered regenerative if sufficient polychromatophils are present. If a robust polychromatophilic response is not present in an anemic patient, it is important to evaluate a blood smear using new-methylene blue stain to detect reticulocytes. Horses do not release polychromatophils in response to anemia (they release macrocytes). Small numbers of polychromatophils may be normal in healthy dogs.