Red blood cell morphology is species-dependent. There are also changes that occur in red blood cells that can give us clues as to underlying diseases. Some of these changes can be pathologic in one context (or in one species) or physiologic in another. For example, small Heinz bodies are commonly seen in the blood of cats without causing anemia. However, Heinz bodies in dog blood are always abnormal and indicate an oxidant-induced hemolytic anemia. In addition, the number or degree of change in the red blood cells are also important. For example, large numbers of spherocytes are typically seen an immune-mediated anemia. Although small numbers of spherocytes can be seen in an immune-mediated anemia, spherocytic-like cells are in low numbers with fragmentation injury and oxidant injury (called pyknocytes in this case). So, all red blood cell changes should be interpreted in the context of the case. For instance, low numbers of schistocytes may be more relevant in a sick dog with concurrent thrombocytopenia and suspect disseminated intravascular coagulation, than in a pre-surgical blood health check in a clinically normal dog. In this section, we will cover the following:
- Species differences in normal red blood cell shape and size. Most mammalian species have round red blood cells, which vary substantially in size. An exception are camelids which have elliptical red blood cells. Species also differ in their regenerative response to an anemia.
- Physiologic and pathologic changes: These can be grouped depending on the change seen. These changes are summarized in our quick guide as well.
- Nucleated RBCs: When you see them and how to identify.
- Change in shape: This includes acanthocytes, drepanocytes, eccentrocytes, echinocytes, elliptocytes, keratocytes, poikilocytes, pyknocytes, schistocytes, spherocytes, stomatocytes, target cells. Changes in red blood cell shapes can give you clues as to the presence of underlying disease or pathophysiologic mechanisms for an anemia. This section provides details on what these shape changes tell us about the animal.
- Acanthocytes, keratocytes and schistocytes: These are seen in anemia and disorders associated with fragmentation injury to red blood cells, such as disseminated intravascular coagulation and hemangiosarcoma in dogs.
- Eccentrocytes, pyknocytes and keratocytes: These indicate oxidant injury to red blood cells. Heinz bodies may also been seen in oxidant injury. Note that keratocytes can be observed in fragmentation and oxidant injury so they are not specific for either of these two types of red blood cell injury.
- Spherocytes: When large numbers are present, they usually indicate an immune-mediated anemia.
- Change in size: This includes macrocytes and microcytes. Changes in red blood cell size in a blood smear really equates to changes in diameter and not changes in volume (as measured by laser- or impedance-based hematologic analyzers).
- Change in color: This includes hypochromasia, polychromasia, and ghost cells. Changes in color help us determine if an anemia is regenerative (polychromasia in all species, other than equine), if there is intravascular hemolysis (ghost cells in a fresh blood sample) or if there is iron deficiency (hypochromasia).
- Change in pattern: Agglutination, rouleaux formation. These help us identify immune-mediated anemia (agglutination) and diseases associated with high fibrinogen (inflammation) or immunoglobulins (immune response, neoplasia affecting B lymphocytes or plasma cells).
- Inclusions: Basophilic stippling, Heinz bodies, Howell-Jolly bodies, siderocytes. These inclusions are used to identify oxidant injury to red blood cells (Heinz bodies), immature red blood cells (basophilic stippling, Howell-Jolly bodies), lead toxicosis (basophilic stippling, siderocytes), and other conditions.
- Quick guide: Table and image summary of RBC changes and terms, what they mean and the conditions they are observed in.