Crenation (Figure 6) can be confused with important RBC changes such as acanthocytosis. Crenation is a shrinking artifact most commonly seen when
less than optimal amounts of blood are collected into the EDTA tube and the peripheral blood film is not made relatively quickly
after the anticoagulation process.4 Some less common causes of crenation include electrolyte disturbances, uremia, and rattlesnake envenomation. A useful differentiating
feature for identifying crenation is that crenation typically affects large numbers of cells in a particular area of the slide,
whereas true poikilocytosis typically affects relatively lower numbers of cells throughout the peripheral blood film. Crenation
can be minimized by preparing blood films immediately after blood samples are collected and properly anticoagulated.
6. Crenation in a dog (modified Wright's stain; 100X). 7. Spherocytes (black arrows) and a polychromatophil (red arrow) in
a dog (modified Wright's stain; 100X). 8. Acanthocytes (arrow) in a dog (modified Wright's stain; 100X). 9. Schistocytes (arrows)
in a dog (modified Wright's stain; 100X). 10. Leptocytes (arrow) in a dog (modified Wright's stain; 100X).
Spherocytes are spherical RBCs that have lost their normal biconcave shape, resulting in more intense staining than normal
RBCs. They have no central zone of pallor, and they appear smaller than normal RBCs (Figure 7). More than four to six spherocytes per 100× field is considered elevated. Spherocytes are commonly seen with many of the
immune-mediated hemolytic anemias we encounter in veterinary medicine. Be careful when attempting to identify spherocytes
in feline RBCs because feline RBCs are much less biconcave than canine RBCs and, therefore, have much less central pallor.
Acanthocytes are abnormally shaped RBCs having two to 10 blunt, fingerlike surface projections of varying sizes (Figure 8). These morphologic changes are related to abnormal accumulation of lipids within the RBC membrane when there is an abnormal
plasma cholesterol:phospholipid ratio. Acanthocytes are seen occasionally in normal animals. Conditions most commonly associated
with acanthocyte formation include underlying metabolic diseases or diseases affecting normal lipid metabolism. Nonneoplastic
and neoplastic (hemangiosarcoma in particular) diseases involving the liver, spleen, and kidney may have associated acanthocytosis
in dogs and, occasionally, in cats.6 Acanthocytosis is most frequently associated with liver disease and splenic hemangiosarcoma.
Schistocytes are RBC fragments formed by mechanical injury (Figure 9). Even in very low numbers (one schistocyte in every three to five 100× objective fields), schistocytes are clinically relevant.
Finding even a few schistocytes may help you identify underlying or subclinical disseminated intravascular coagulation (DIC).
Microvascular mechanical fragmentation of RBCs associated with diseases such as hemangiosarcoma and dirofilariasis may also
result in schistocyte formation.7
A leptocyte (codocyte, target cell) is an RBC with excess cell membrane that forms a shape often referred to as having a Mexican
hat appearance (Figure 10). Leptocytes are seen occasionally in normal animals. Increased numbers of leptocytes (> 3/100× oil-immersion field) are expected
with polychromasia because polychromatophils have excess membranes compared with normal RBCs. Consequently, leptocytosis is
common when reticulocytosis is present, so most reference laboratories do not report this morphologic finding. However, laboratories
will report leptocytes when they are seen in the absence of polychromasia since the excess lipid membrane compared with cytoplasmic
volume is an abnormality. The two primary mechanisms causing this abnormal morphology are either an upset in the cholesterol:phospholipid
ratio in plasma resulting in lipid loading (as might be seen with liver disease and other metabolic disorders) or a decrease
in cytoplasmic content compared with normal (as might be seen with iron deficiency typically due to chronic blood loss).8 With iron deficiency, RBC hemoglobin content decreases, and in addition to the leptocytosis, hypochromasia is often observed.