The white cell count (WBC) is the total number of leukocytes in a volume of blood, expressed as thousands/µl. As with the RBC, the WBC can be done by manual methods or by automated cell counters. The WBC by any method is a count of nuclei or total nucleated cell count. If nucleated red blood cells (nRBC) are circulating in blood, they will be included in the nucleated cell count whether the count is done by manual methods or by automated analyzers. In such cases, the WBC represents the leukocyte count only after it has been corrected for the nucleated red cells (nRBCs). The number of nRBCs per 100 leukocytes is recorded during the differential leukocyte count. Then a correction is made as follows:

Manual WBC counting
The manual procedure, using a diluting chamber and a hemocytometer, is no longer used except for white cell counts in body fluids with extremely few cells (< 1,000 cells/µL), e.g. cerebrospinal fluid. It involves diluting blood in a diluent that lyses the red cells to remove them from view. A hemocytometer is charged with the diluted blood, and nuclei are counted in the appropriate areas of the grid using a light microscope. The manual WBC is a reasonable test to do in-office since it is not terribly time- and labor-intensive and gives acceptably accurate results.

Impedance counters
Laboratories use either impedance-based electronic cell counters (see RBC section) to generate WBC counts in body fluids and non-EDTA samples. In our laboratory, the Coulter Z2 (a bench-top impedance counter) is our back-up analyzer. We use this cell counter when our Advia is out of commission or a sample other than EDTA-anticoagulated blood is submitted for cell counts. This is the standard instrument used for cell counts in body fluids, other than blood, e.g. joint fluids, peritoneal fluids, because these fluids cannot be analyzed through the Advia.

Flow cytometry counters
We use the hematology analyzer, the Advia, to obtain WBC counts. This analyzer counts WBC in 2 ways, both of which use flow cytometry. In the first method called the peroxidase method, the white cells are stained with peroxidase and the cells are counted based on size and staining characteristics.

A. Neutrophils B. Monocytes C. Lymphocytes D. Eosinophils E. Large unstained cells F. Platelets/noise G. Debris/noise

This method also provides an automated differential cell count by separating the cells into clusters (see image on left for a dog peroxidase cytogram). The second method, called the basophil method, involves stripping the cells of cytoplasm and counting nuclei (see image below). In this cytogram, the cells are displayed as a worm, with the mononuclear cells (lymphocytes and monocytes) in the head and the granulocytes (neutrophils) in the body of the worm. This is a far more accurate method for counting white cells and is the default method on the Advia. The peroxidase method will give falsely high counts in samples with numerous platelet clumps. This type of artifact is also prominent with impedance-based counters, resulting in erroneously high WBC counts in certain species, especially cats (whose platelets clump at the drop of a hat).

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