In the previous article, we discussed how to examine the erythron and thrombon components of peripheral blood films. In this article, we again use a question-based format to guide you in evaluating the leukon by assessing white blood cell (WBC) numbers and morphology. All five WBC cell types are assessed. Table 1 lists the general patterns of WBC response under a variety of circumstances.
Table 1 Common Leukocyte Patterns in Dogs and Cats
Is the total WBC count elevated, normal, or decreased?
Experience is required to accurately estimate cell counts directly from blood films. Subjective analysis of total WBC numbers may be performed by counting three to five 20X objective fields; 10 to 20 WBCs/20X field is considered normal in dogs and cats. Another method involves counting several 100X oil-immersion monolayer fields and multiplying the average number of WBCs/100X oil-immersion field by 2,000 to obtain a final estimated total WBC count.1
Is a left shift present?
A left shift may be the only indicator of active inflammation in veterinary patients because total WBC and neutrophil counts are frequently within the reference range. Left shifts are characterized by increased numbers of immature neutrophils (e.g. band cells, metamyelocytes) in circulation. A left shift is a hallmark of inflammation, so accurately identifying band cells is extremely valuable. No hematology analyzer (in-house or reference laboratory instrumentation) has been documented to accurately identify band neutrophils; consequently, they must be identified microscopically. On blood films, the nucleus of a band neutrophil typically has parallel sides (Figure 1), whereas the nucleus of a mature neutrophil is distinctly segmented. One useful approach to differentiate band neutrophils is to estimate the degree of nuclear indentation. First, identify the narrowest and widest portions of the nucleus. If the narrowest portion is less than one-third of the widest portion, the cell is classified as a band cell.1
Figure 1. Canine band neutrophils (modified Wright's stain; 100X). Figure 2. A canine monocyte (modified Wright's stain 100X). Figure 3. A buffy coat smear from a dog showing a macrophage (arrow) phagocytizing Histoplasma species (modified Wright's stain 100X). Figure 4. A reactive lymphocyte in a dog (modified Wright's stain 100X).
Is there a monocytosis?
The monocyte-macrophage continuum represents the second major branch of the circulating phagocyte system (neutrophils are the first). Monocytes (Figure 2,), unlike granulocytes (neutrophils, eosinophils, basophils), are released into the peripheral blood as immature cells and then differentiate into phagocytic macrophages (Figure 3), epithelioid macrophages, or multinucleated giant cells.
Monocytosis can be another indicator of inflammation. It may be seen in both acute and chronic conditions but may also be a component of stress leukograms. Conditions frequently associated with monocytosis include systemic fungal diseases (histoplasmosis, blastomycosis, cryptococcosis, coccidioidomycosis, aspergillosis), immune-mediated hemolytic anemia, bacterial endocarditis, certain neoplastic diseases (particularly when tissue necrosis is noted), and certain pyogranulomatous diseases (e.g. feline infectious peritonitis, toxoplasmosis, foreign body reactions).2 The best interpretation of monocytosis in the absence of a stress leukogram is tissue demand for macrophages.
Is a persistent eosinophilia present?
Persistent eosinophilia indicates a systemic hypersensitivity reaction and can be another indicator of inflammation. Conditions associated with persistent eosinophilia include heartworm disease in dogs and cats, feline asthma, canine atopic syndromes, feline eosinophilic granuloma complex (linear plaque form), hypereosinophilic syndrome, mast cell leukemia, and flea allergy dermatitis.3 Parasitic infections confined to the bowel, such as whipworm infections or ascarid and hookworm infections in adult nonpregnant dogs and cats, do not cause persistent peripheral eosinophilia because the parasites do not have a systemic phase.4