Page 1399 - Small Animal Internal Medicine, 6th Edition

CHAPTER 85
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Leukopenia and

Leukocytosis

GENERAL CONSIDERATIONS A differential WBC count may be reported in relative
(percentages) or absolute numbers (number of cells per
The leukogram, evaluated as part of the complete blood microliter). However, the absolute leukocyte numbers, not
count (CBC), includes a quantification of the total number the percentages, should always be evaluated because the
of white blood cells (WBCs) and the differential WBC count; latter may be misleading, particularly if the WBC count is
ideally, it should also include morphologic leukocyte evalu- very high or very low. For example, a total WBC count of
ation. Although a specific disorder is rarely diagnosed on the 3000 cells/µL (or 3 × 10 /L) and a differential WBC count of
9
basis of the leukogram, the information obtained may be 90% lymphocytes and 10% neutrophils can lead to one of the
useful in limiting the number of differential diagnoses or in following two conclusions:
predicting the severity of the disease and its prognosis. In
addition, sequential leukograms are helpful in monitoring a 1. Based on the percentages alone, the dog has lymphocyto-
patient’s response to therapy. sis and neutropenia; in this situation, the clinician may
According to standard laboratory techniques, all nucle- erroneously focus on the lymphocytosis rather than the
ated cells are counted during a WBC count, including neutropenia.
nucleated red blood cells (nRBCs). Differential leukograms 2. Based on the absolute numbers, the dog has severe neu-
determined by particle counters used at human referral labo- tropenia (300 cells/µL) with a normal lymphocyte count
ratories are typically not valid for cats and dogs. New vet- (i.e., 2700 cells/µL).
erinary benchtop analyzers provide reliable WBC total and
differential counts. The ProCyte Dx provides a five-part dif- The latter obviously reflects the actual clinical situation.
ferential WBC count (neutrophils, lymphocytes, monocytes, The clinician should then concentrate on determining the
eosinophils, and basophils) and includes flags for nRBCs and cause of the neutropenia and ignore the normal lymphocyte
left shift or toxic neutrophils, whereas the impedance-based count.
analyzers provide a three- or five-part differential count.
As a general rule, when a benchtop hematology analyzer
yields values outside the reference interval (RI) or the values NORMAL LEUKOCYTE MORPHOLOGY
are flagged, the clinician or a technician should carefully AND PHYSIOLOGY
examine the graphics (see Figs. 79.6, 80.4, 80.6, 82.1, 82.3,
and 82.9) and a blood smear. From a morphologic standpoint, leukocytes can be classified
Leukocytosis occurs if the WBC count exceeds the upper as polymorphonuclear or mononuclear. Polymorphonuclear
limit of the RI for the species; leukopenia occurs if the WBC cells include the neutrophils, eosinophils, and basophils; the
count is below the RI. In some breeds of dogs (e.g., Belgian mononuclear cells include the monocytes and lymphocytes.
Tervuren, Greyhound), the WBC and neutrophil counts are Their basic morphologic and physiologic characteristics are
frequently below the RI for the species, thus resulting in outside the scope of this chapter.
an erroneous diagnosis of leukopenia and neutropenia in an The following morphologic changes have important clini-
otherwise healthy dog. This should be kept in mind in dogs cal implications and should thus be recognized:
undergoing chemotherapy (see Chapters 75 and 76) because
treatment delays based on a low WBC or neutrophil count 1. Neutrophils may become toxic in response to injury (Fig.
(normal for the breed) have a detrimental effect on the 85.1); toxic neutrophils display characteristic cytoplasmic
patient’s well being. changes, including basophilia or granulation, vacuolation,

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