Page 315 - WSAVA2017
P. 315

A.R. Alleman1
1Lighthouse Veterinary Consultants, LLC, Gainesville, FL 32606
The evaluation of a blood smear will allow the practitioner to gain rapid, valuable information regarding the health
of the patient when the evaluation is performed in a systematic fashion. The important clinical information needed for the hematologic evaluation of an animal can all be obtained by estimating cell numbers and evaluating the morphologic changes in erythrocytes, leukocytes and platelets. The value of these findings, many of which are not recognized by automated cell counters, cannot be overemphasized.
Blood Collection and Slide Preparation
Vacutainer tubes containing EDTA should be filled to the designated amount. Partial filling of vacutainer tubes with blood may cause artifactual changes in cell morphology and numerical values. Blood smears should be prepared as quickly as possible in order to minimize artifactual changes in erythrocytes and leukocytes such as red cell crenation, leukocyte vacuolation and nuclear swelling
and pyknosis. In addition, prolonged exposure to EDTA may make it more difficult, or even impossible to identify infectious agents, such as Mycoplasma haemofelis (formerly Haemobartonella felis), in the blood of infected cats. The coverslip technique for making smears is preferred over the glass slide technique. This technique minimizes traumatic injury to cells during slide preparation. This technique produces a more even distribution of
cells, allowing more accurate estimation of leukocyte and platelet numbers. Smears should be rapidly dried with a blow drier to eliminate artifacts of air-drying red blood cells. This is particularly important when attempting to identify red cell parasites such as haemoplasmas or evaluation of erythrocyte shape changes.
Scanning the Smear
The first step in the evaluation of a blood smear is
to scan the slide using a 10X or 20X objective. With regard to the red blood cells, we should observe red cell density and presence of rouleaux or agglutination. (Rouleaux may be differentiated from agglutination by saline test where 1 drop of blood is mixed with 0.5 to 1.0 ml of physiological saline solution and observed
on wet mounts, unstained. Rouleaux will disperse
with saline dilution.). With regard to nucleated cells
we should confirm that the mature neutrophil is the predominant cell type. The presence of any left-shifted neutrophils or large or atypical leukocytes, as well as the presence of any nucleated erythrocytes should also be recorded. Platelet clumps should also be identified at this magnification because they will affect how we interpret platelet numbers later in the evaluation. This is particularly crucial in the evaluation of the feline blood film because platelet clumping is a common occurrence in this species. Leukocyte numbers may be estimated using the following formula. Formula: # cells/:l = (Ave.
# of cells per field) X (Objective power)2. The objective used to estimate leukocyte numbers should be one where approximately 5 - 10 leukocytes are seen per field. Example: If an average of 5 cells were counted for each 50X field, the total leukocyte count would be (5) X (2,500) = 12,500 cells / :l.
Erythrocyte Evaluation
Erythrocyte evaluation begins with the search for agglutination or rouleaux formation using a scanning objective. Erythrocyte morphology should be evaluated using the 100X, oil immersion lens in an area of the smear where red cells are evenly spaced, usually slightly behind the feathered edge. Red blood cells are evaluated for changes in size, shape, color and inclusions. Erythrocytes are normally very uniform in size. Typically, there is minimal variation in the size, shape or color of
the erythrocytes in the blood smear. In most domestic species, red cells are normally a biconcave disc shape (discocyte). However, the central pallor (a paler staining area in the center of the erythrocyte) that is normally prominent in the blood cells of dogs of often not seen in the cat. This is associated with the smaller erythrocyte size in the cat (5-6 μm) than in the dog (7 μm). Therefore, in the feline species, spherocytosis (small, round, dense cells with no central pallor) cannot reliably be identified by examination of a blood film. Spherocytosis can only reliably be identified in the canine.
Anisocytosis is defined as a variation in cell size. This usually indicates the presence of abnormally large erythrocytes (macrocytes) which are commonly seen in regenerative anemias. Macrocytes and anisocytosis may be seen in nonregenerative anemias in cats with FeLV infection and some preneoplastic (dyplastic)
and neoplastic (leukemias) diseases. Animals with regenerative anemias from any cause typically have marked anisocytosis due to the large polychromatophilic erythrocytes present. Dogs with IMHA usually have marked anisocytosis due to the presence of small spherocytes and large polychromatophilic cells.
An Urban Experience

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