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

Classifying Anemias in Dogs and Cats
Regenerative anemias can be further classified as either blood loss or hemolytic (decreased RBC lifespan) anemias. Whenever you suspect hemolysis, closely examine RBC morphology to detect spherocytes, acanthocytes, schistocytes, RBC inclusions, and parasites (see below). Figure 3 is a flow chart for classifying anemias in dogs and cats .

Are nucleated RBCs present?

4. A nucleated RBC (arrow) in a dog (modified Wright's stain; 100X). 5. A saline wet preparation of canine blood showing autoagglutination (20X).
Nucleated RBCs, or metarubricytes (Figure 4), are not found in any appreciable numbers in mammalian peripheral blood samples. Most clinicians consider fewer than 4 nucleated RBCs/100 WBCs to be insignificant when the WBC count is within the reference range. Nucleated RBCs may be seen in increased numbers with strongly regenerative anemias, but they should always be less numerous than the polychromatophils or reticulocytes. This finding has been identified as an appropriate nucleated RBC response by some clinicians.

An inappropriate nucleated RBC response occurs when more than 5 nucleated RBCs/100 WBCs are present in the absence of polychromasia. Conditions associated with an inappropriate nucleated RBC response include bone marrow stromal damage, extramedullary hematopoiesis, fractures, hyperadrenocorticism, feline leukemia virus infection, chemotherapeutic drug administration, and lead toxicosis, among others.5 Splenic dysfunction (e.g. decreased clearance of circulating nucleated RBCs) is an important cause of an inappropriate release of nucleated RBCs and may occur in patients in which the spleen has been removed and in patients with splenic neoplasms, especially hemangiosarcoma.

Is autoagglutination present?

Agglutination is unorganized three-dimensional clumping of RBCs that must be differentiated from rouleau formation. It is common in dogs with immune-mediated hemolytic anemia. When confirmed, autoagglutination suggests an immune-mediated process such as autoimmune-mediated hemolytic anemia or drug-induced hemolytic anemia (e.g. cephalosporins, penicillin) because RBC surface antibodies cause cell cross-linking and the resulting agglutination.

Rouleau formation is organized linear arrays of RBCs caused by decreased zeta potentials from plasma proteins (e.g. globular proteins, fibrinogen) coating RBCs. This distinctive formation is commonly described as stacks of coins. Unlike agglutination, which is a strong cross-linking between cells, rouleau is a result of a weak binding between cells because of dissimilar ionic charges on the RBC surface. Rouleau formation is a common finding when fibrinogenesis is increased and must be differentiated from agglutination. On routine Wright's-stained or modified-Wright's-stained blood films, marked rouleau formation and agglutination may be difficult to differentiate.

Whenever you suspect agglutination on a blood film, mix a drop of the well-mixed EDTA anticoagulated blood on a new glass slide with two or more drops of isotonic saline solution. Add a cover slip, and view the mixture as an unstained wet preparation. Under these conditions, most rouleaux formations dissipate but autoagglutination typically persists and is recognized as clumped RBCs (Figure 5).

Are poikilocytes present?

Normal canine RBCs are shaped like biconcave disks with prominent central pallor. Feline RBCs have much less apparent central pallor. Abnormally shaped RBCs are called poikilocytes and include artifactual changes (crenation) as well as true abnormalities (e.g. spherocytes, acanthocytes, schistocytes, leptocytes).


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