Abstract<\/strong><\/p>\nBlood smear review is a key component of a CBC and facilitates identification of inclusions (e.g., parasites, Heinz bodies, basophilic stippling) and erythrocyte morphology changes that may not be detected by an automated hematology analyzer. Some infectious agents\/infected cells, such as microfilaria and schizont-filled macrophages, can be identified on low magnification (10\u00d7 for large structures), while detection of others requires high magnification (100\u00d7 for small structures). Infectious inclusions need to be differentiated from artifacts and noninfectious inclusions that can indicate other physiologic, metabolic, or pathologic processes. Multiple erythrocyte morphology changes (e.g., association, size, shape, color, inclusions) can be grouped by mechanism and correlated with other clinical\/diagnostic findings to determine differential diagnoses (e.g., iron deficiency, oxidative injury, immune targeting) and prioritize next clinical steps.<\/p>\n
Take-Home Points<\/strong><\/p>\n\n- Evaluation of a freshly prepared blood smear can facilitate detection of various erythrocyte morphology changes, including infectious and noninfectious inclusions; therefore, when sending blood to a reference laboratory for analysis, always submit a freshly prepared blood smear along with an anticoagulated sample.<\/li>\n
- Manually performing a blood smear review to assess automated hematology instrument flags is recommended because some morphology changes can interfere with automated analytes (e.g., agglutination, nucleated red blood cells).<\/li>\n
- Analyzing a blood smear involves conducting a low-magnification scan of an entire smear, including the feathered edge, to look for large extracellular organisms and infected cells and then moving to high magnification to evaluate the feathered edge and monolayer for small erythrocyte inclusions.<\/li>\n
- Erythrocyte inclusions and changes must be distinguished from artifact; correlated with clinical findings; and, in most cases, confirmed by molecular testing because light microscopy sensitivity for detecting infectious agents can be limited.<\/li>\n
- Evaluating erythrocyte abnormalities along with other blood work data and clinical history may help elucidate differential diagnoses.<\/li>\n
- Bone marrow can take up to a week to mount a response to anemia; thus, if the duration of anemia is unknown, it may be difficult to initially distinguish preregenerative and nonregenerative anemia and serial monitoring, with or without reticulocyte enumeration, may be helpful.<\/li>\n<\/ul>\n
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Blood smear review adds crucial complementary information to routine hematology analyzer results. (Normal blood smear findings and erythrocyte association, size, color, and shape changes in dogs and cats are discussed elsewhere.1,2<\/sup>) This article describes how to identify erythrocyte inclusions and organize erythrocyte findings into broad categories to determine differential diagnoses. As a reminder, start with a properly prepared and stained blood smear. Scan the entire smear on low magnification (10\u00d7 to identify large structures\/cells), and then view the monolayer and feathered edge on high magnification (100\u00d7 to discern smaller changes\/structures).1<\/sup><\/span><\/p>\nErythrocyte Precursors and Maturation<\/h2>\n
To identify and understand erythrocyte morphology changes, knowing the stages of erythropoiesis is necessary. Erythroid lineage includes the following cells from least to most mature:<\/span><\/p>\n\n- Rubriblast (<\/b>FIGURE 1A<\/b><\/span>):<\/b> Large cell with scant, deeply basophilic cytoplasm surrounding finely stippled nuclear chromatin and multiple prominent nucleoli.<\/li>\n
- Prorubricyte (<\/b>FIGURE 1A<\/b><\/span>):<\/b> Similar to rubriblast but lacks distinct nucleoli.<\/li>\n
- Rubricyte (<\/b>FIGURE 1A AND 1B<\/b><\/span>):<\/b> Smaller than earlier precursors with similar scant cytoplasm, which can be dark (basophilic) to light (polychromatophilic), surrounding coarsely clumped nuclear chromatin and indistinct nucleoli.<\/li>\n
- Metarubricyte (<\/b>FIGURE 1A<\/b><\/span>):<\/b> More cytoplasm than earlier precursors, similar size and cytoplasmic color as later precursors, condensed nuclear chromatin.<\/li>\n
- Polychromatophil (i.e., reticulocyte, <\/b>FIGURE 1B<\/b><\/span>):<\/b> Purple\/blue-tinged anucleate cell that is larger than a mature erythrocyte and represents less than 1.5% of circulating erythrocytes in healthy dogs and cats.<\/li>\n
- Mature erythrocyte (<\/b>FIGURE 1B<\/b><\/span>):<\/b> Round anucleate cell with variably prominent central pallor depending on species.<\/li>\n<\/ul>\n
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