Three-minute peripheral blood film evaluation: The erythron and thrombon - Veterinary Medicine
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Three-minute peripheral blood film evaluation: The erythron and thrombon

Are RBC inclusions present?

Accurately characterizing various inclusions is an important aspect of blood film evaluation. Inclusions may include Heinz bodies, basophilic stippling, Howell-Jolly bodies, and certain infectious agents.

11. Heinz bodies which are noselike RBC projections (arrows) in a cat (modified Wright's stain 100X).12. A blood film from a cat showing Heinz bodies they are easier to see with new methylene blue stain (100X). 13. Basophilic stippling (black arrow) and a polychromatophil (red arrow) in a dog (modified Wright's stain 100X). 14. Babesia canis (arrow) in a dog (modified Wright's stain 100X). 15. Mycoplasma haemocanis (arrow) in a dog (modified Wright's stain 100X).
Heinz bodies are localized accumulations of denatured, oxidized, and precipitated hemoglobin in the RBC. They often affix to the inner RBC membrane and project from the RBC surface. Conditions associated with Heinz body hemolytic anemia include acetaminophen toxicosis in cats and acute onion and zinc toxicosis in dogs. Diabetes mellitus, hyperthyroidism, and lymphosarcoma in cats and the use of oral benzocaine sprays are also associated with increased numbers of Heinz bodies; but anemia is not present in these cases.8 The Heinz bodies observed in cats may be small or not project from the cell surface, making their identification difficult (Figure 11). New methylene blue staining will help confirm the presence of Heinz bodies; the Heinz bodies stain dark blue compared with the rest of the erythrocyte (Figure 12). In contrast to Heinz bodies in dogs, Heinz bodies can be an incidental finding in cats, and their relevance must be determined by evaluating clinical signs and other hemogram parameters.

Basophilic stippling is seen in Romanowsky-stained films as small, dark-blue punctate aggregates of residual ribosomes in RBCs (Figure 13). It has traditionally been associated with lead toxicosis in dogs but may also be associated with highly regenerative anemias in any species. Further diagnostic investigation is important when basophilic stippling is seen in the absence of reticulocytosis.

Howell-Jolly bodies are small nuclear remnants that may be increased with accelerated RBC regeneration. In healthy animals, the low numbers of RBCs with Howell-Jolly bodies released from the bone marrow are quickly removed from circulation primarily through the action of fixed tissue macrophages in the spleen. If Howell-Jolly bodies are easily identified and no reticulocytosis is noted, investigate underlying splenic disease. This inclusion can be an incidental finding in cats because of the open splenic architecture and decreased erythrophagocytic properties inherent to the feline spleen.

Infectious agents including Babesia (Figure 14), Cytauxzoon, and Mycoplasma (previously known as Haemobartonella) species may be identified in RBCs. Evaluation for Mycoplasma species (Figure 15) should be performed on freshly collected blood samples that have not been refrigerated to improve identification.


Are platelet numbers normal, decreased, or increased?

16. A blood film from a cat showing clumped platelets (arrow) (modified Wright's stain; 20X).
Microscopic validation of platelet counts is an important component of blood film evaluation. Platelet clumping (Figure 16) interferes with accurate enumeration of platelets with all hematology analyzers and with manual counting methods. Clumping occurs in many samples because of platelet activation during the collection process. Increased numbers of large platelets may also result in inaccurate platelet counts when impedance counting methods are used.

On a well-prepared peripheral blood film in which no marked platelet clumping at the feathered edge is present, the average number of platelets observed per 100 oil-immersion monolayer field multiplied by 20,000 provides a good estimate of the number of platelets/l. As a general rule, you should see a minimum of eight to 10 platelets and a maximum of 35 to 40 platelets per 100 oil-immersion monolayer field of view.9


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