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An Urban Experience
Poikilocytes are abnormally shaped cells (See Attached Figure). There are several different types of poikilocytes each with a different specific cell morphology. Different types of poikilocytes suggest the occurrence of specific disease processes (See chart on last page). Most poikilocytes are due to pathologic changes in erythrocytes, however, echinocytes (crenated red cells) may be formed iatrogenically from insufficient blood in EDTA tubes or delay in slide preparation from collected blood. Low numbers of echinocytes are often seen in the peripheral blood of cats. Two of the most commonly observed shape abnormalities are schistocytes and acanthocytes. Schistocytes are small, irregularly shaped red cell fragments (Figure above). They result from mechanical trauma to circulating erythrocytes and, thus, are considered the hallmark of red cell fragmentation. In dogs, they are most frequently seen with DIC, microangiopathic hemolytic anemia, and neoplasms, particularly HSA; the latter can result in both DIC and microangiopathic hemolytic anemia. Schistocytes
have been identified in 25% to 50% of dogs with
HSA. However, other conditions, such as congestive heart failure, glomerulonephritis, myelofibrosis, chronic doxorubicin toxicosis, and increased red cell fragility associated with severe iron deficiency, may also result
in the formation of schistocytes. Acanthocytes, or spur cells, are irregularly shaped erythrocytes containing membrane spicules that are unevenly distributed around the red cell surface. Unlike schistocytes, however, acanthocytes have not been fragmented and are
similar in size to normal erythrocytes. These cells result from alterations in the cholesterol and/or phospholipid concentration in the red cell membrane and are seen
in dogs with severe iron deficiency anemia, diffuse liver disease, portocaval shunts, high-cholesterol diets, or HSA. The mechanism underlying acanthocyte formation in dogs with HSA is not completely understood. Coexistence of acanthocytes and schistocytes in a dog with anemia is highly suggestive of HSA.
The density of the red color in erythrocytes is dependent on the concentration of hemoglobin. Changes in hemoglobin concentration can be in the form of polychromasia or hypochromasia. Polychromasia
is defined by the presence of basophilic appearing erythrocytes. Polychromatophilic erythrocytes are seen in regenerative anemias when immature erythrocytes with decreased hemoglobin and increased amounts of RNA are released from the bone marrow. Numbers of polychromatophilic cells correlate well with numbers
of reticulocytes. There are two types of reticulocytes
in feline blood, aggregate reticulocytes and punctate reticulocytes (these are discussed in more detail below). Polychromasia or the number of reticulocytes present is the only peripheral blood findings that can be used to determine if an anemia is regenerative or not. Erythrocytes are termed hypochromic if they stain
less intensely red than normal. Generally, the hemoglobin in hypochromic patients is concentrated around the periphery of the cells causing a larger than normal central pallor. Normal canine erythrocytes have a central pallor with a diameter that is equal to approximately one-
third of the diameter of the cell. In iron deficient dogs, the central pallor of most of the erythrocytes is half the diameter of the cell or greater, allowing the morphologic identification of hypochromasia. Hypochromatic erythrocytes result from decreased hemoglobin,
most often associated with iron deficiency. Most iron deficiencies in domestic animals are due to chronic blood loss. Normal feline erythrocytes have a very small or no central pallor. In iron deficient cats, the central pallor
is visualized in most, if not all of the erythrocytes and usually occupies 1/3 to 1⁄2 the cell diameter, allowing
the morphologic identification of hypochromasia. Most (>95%) of all iron deficiencies are the result of prolonged or chronic blood loss. This is typically due to either external parasitism or GI neoplasms.
Platelet Evaluation
Platelet evaluation begins with the search for clumps using a scanning objective (Figure upper right). If platelet clumps are observed, quantitative assessment of platelets will be falsely reduced. An estimation of platelet numbers is done using the 100X oil emersion lens. The formulas for estimation of platelet numbers are (Dog): # Cells / μl = (# cells per 100X oil field) X 15,000, and for the (Cat): # Cells / μl = (# cells per 100X oil
field) X 20,000. The technique of platelet evaluation is particularly useful in evaluating platelets in the cat since automated cell counts are often unreliable in assessing platelet numbers for this species. The overlap in platelet and erythrocyte size, along with the propensity for feline platelets to clump or aggregate makes it very difficult to obtain accurate platelet counts using most automated hematology analyzers. Megaplatelets (Figure lower
right), platelets as large as or larger than erythrocytes, may indicate platelet regeneration due to a peripheral destruction or consumption of platelets. However, in
the cat, platelets are often larger than erythrocytes due to both the larger size of the platelet (than in dogs) and the smaller size or the erythrocyte. Thrombocytopenia may result from a production problem in the marrow, or a loss in the peripheral circulation due to destruction or consumption of platelets. A bone marrow evaluation may be necessary to make this distinction.
Leukocyte Evaluation
Leukocyte evaluation begins using a scanning objective by observing the mature neutrophil as the predominant cell type and identifying the presence of immature neutrophils (bands, metamyelocytes or myelocytes) or reactive changes in monocytes and lymphocytes. Large, immature blast cells should also be identified at this time. These pathological changes and other changes

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