Automated hematology analyzers are frequently used to evaluate erythrocytes and detect changes of concentration, size, and volume; however, analyzers are often not reliable for detecting changes of erythrocyte morphology that may have a clinical effect. For example, changes of associations among erythrocytes could suggest an underlying inflammatory or immune-mediated disease; erythrocyte size and color changes could indicate a regenerative response or iron deficiency; and certain erythrocyte shape changes could point to erythrocyte damage\/fragmentation, immune-targeting, or oxidative injury.<\/p>\n
Take-Home Points<\/strong><\/p>\n Automated hematology analyzers help clinicians identify abnormalities of erythrocyte and hemoglobin concentrations as well as certain erythrocyte morphology changes (e.g., size, volume). Automated analyzers are helpful for diagnosing and classifying anemia; however, they fail to fully assess erythrocyte morphology, which can provide useful diagnostic information about the underlying cause of anemia or other diseases. In particular, certain abnormalities of erythrocyte shape (e.g., acanthocytes, elliptocytes, codocytes, eccentrocytes) and association (e.g., agglutination, rouleaux) may not be detected by current automated technology, leading to possible misclassification of cells and\/or false size and volume assessments. Recognizing abnormalities through blood smear review can identify pathologic conditions and provide critical information about the underlying mechanisms of disease. <\/span><\/p>\n This article reviews the most common erythrocyte morphology changes that can be identified by blood smear review and focuses on association, size, color, and shape (<\/strong><\/span>BOX 1<\/span><\/strong>)<\/strong>. An article in the Today\u2019s Veterinary Practice<\/i> November\/December 2025 issue will cover erythrocyte inclusions (e.g., Heinz bodies, Howell\u2013Jolly bodies, infectious agents).<\/span><\/p>\n Size<\/strong><\/p>\n Color<\/strong><\/p>\n Shape <\/strong><\/p>\n <\/div><\/div>\n In dogs and cats, mature erythrocytes are rounded, anucleate cells that consist of a plasma membrane and underlying cytoskeleton, hemoglobin, and glycolytic enzymes (<\/strong><\/span>FIGURE 1<\/span><\/strong>)<\/strong>. They principally deliver gases to and from tissues via capillaries, which are not much larger than the diameter of an erythrocyte. As a result, the erythrocyte membrane and cytoskeleton maintain a biconcave disc shape that enables flexibility to move through the vessels without rupturing and to provide efficient gas exchange. Hemoglobin is an iron-containing protein that is responsible for binding oxygen and carbon dioxide for transport; it makes up roughly one-third of an erythrocyte’s contents (hence the reason why a patient\u2019s hemoglobin should be one-third of its hematocrit) and also provides some structural support. Because mature erythrocytes lack organelles, they rely on glycolytic enzymes for energy production as well as antioxidants to protect against oxidative damage during oxygen exchange. Alterations to these cellular components can lead to morphology changes. The general approach to evaluating a blood smear and normal erythrocyte morphology has been described elsewhere.1<\/sup><\/span><\/p>\n\n
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Erythrocyte Basics<\/h2>\n
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