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Leukocytosis and Leukopenia: Overview |  |
| Leukocytes, or white blood cells (WBC), in mammalian blood include segmented neutrophils, band neutrophils, lymphocytes, monocytes, eosinophils, and basophils. These cells vary in their site of production, their duration of peripheral circulation and recirculation, and the stimuli that affect their release into and migration out of the vascular bed. Differential counts also vary among species. |
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Leukocytosis is an increase in the total number of circulating WBC; leukopenia is a decrease. Changes in WBC counts and morphologic appearance of various leukocytes are evaluated by comparison with reference ranges for each species. Differential WBC counts may be reported either as total (absolute) cell numbers per volume of blood (µL) or in relative percentages of the total. Valid interpretations can be made only by considering the absolute numbers.
See
table 6, Table: Hematologic Reference Ranges, for reference values for total WBC and differential WBC counts in absolute numbers and in percentages for common domestic species. In neonates the total WBC count is more variable and often higher than in adults. Age-related reference values should be used to evaluate hemograms in young animals, especially species in which lymphocytes are more numerous (and neutrophils less) than in adults. This may interfere with the
identification of lymphopenia. Generally, differential WBC patterns of adults are reached at about the age of sexual maturity. |
| Differential WBC counts are performed by identifying and classifying the first 100 or 200 intact WBC encountered in the monolayer of a blood smear. The absolute number of each WBC type is then determined by multiplying the percentage of a particular WBC in the differential count by the total WBC count. Smears from buffy coat preparations are sometimes used to concentrate cells for differential counts in cases of marked leukopenia, to find circulating neoplastic cells, to identify
intracellular infectious agents, or to perform fluorescent antibody testing. |
| Increases in any of the various cell types are indicated by the suffix “philia” or “cytosis” (eg, neutrophilia, lymphocytosis, etc); decreases are indicated by the suffix “penia” or “cytopenia” (eg, neutropenia, lymphopenia, etc). An increase in immature nonsegmented neutrophils above species reference values is called a
left shift, which may be regenerative or degenerative. A regenerative left shift occurs when a leukocytosis is due to a neutrophilia and the absolute number of nonsegmented neutrophils does not exceed the absolute number of segmented neutrophils. A degenerative left shift occurs when nonsegmented neutrophil numbers exceed segmented neutrophil numbers. Occasionally, marked peripheral leukocytosis is difficult to differentiate from granulocytic leukemia due to the
magnitude of both the left shift and increase in WBC. When the total WBC count exceeds 30,000/µL in horses and cows, and 75,000/µL in dogs and cats, with a neutrophilia and left shift due to inflammation, these reactions may be termed a
leukemoid response. |
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Although nucleated red blood cells (nRBC) are counted as WBC by most counting techniques, they should not be included in the WBC differential count. The total nucleated cell count must be corrected to give an accurate WBC count if >5 nRBC/100 cells are encountered during the WBC differential count. The total WBC count must be corrected for nRBC by the following formula: |
| Within blood vessels, WBC occur in 2 subpopulations—the central and the marginal pools. In most species, the ratio of neutrophils in the central pool to neutrophils in the marginal pool is 1:1; however, in cats it is 1:3. WBC in venipuncture samples represent the central pool because venipuncture fails to collect the WBC of the marginal pool along the endothelial surfaces. WBC in the circulating pool can be increased by the following mechanisms: epinephrine can redistribute
neutrophils from the marginal pool to the circulating pool, and corticosteroids can prevent neutrophils from adhering to the endothelial surface. |
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