Interpretation of the Ieukogram in the context of the clinical history, the physical examination, and the rest of the minimum laboratory database (complete blood cell count, serum biochemical data, and urinalysis) can provide valuable insights into the diagnostic process in large animal species.
Often leukogram data are used to evaluate patients for the presence of inflammation or hematopoietic neoplasia. Careful attention to sample collection and handling (see Chapter 23) will help to ensure high-quality results.
Key components of the leukogram include the total white blood cell (WBC) count, differential cell counts including the absolute numbers of each leukocyte type, and a microscopic evaluation of the blood film. Blood film evaluation is necessary to visually verify the WBC count, to generate the differential, and to evaluate the leukocytes for important morphologic abnormalities that contribute to the diagnostic interpretation of the quantitative data. Although the newer automated hematology analyzers will generate leukocyte differentials for numerous species, the Clinical and Laboratory Standards Institute still considers the manual differential cell count to be the reference standard.1 Microscopic examination of the blood film for the presence of toxic change, band neutrophils, reactive changes in lymphocytes, neoplastic and atypical cells, and organisms as well as for validation of the total leukocyte count is required for a full characterization of the leukogram. For instance, in cases of equine acute lymphoid and myeloid leukemia, only small numbers of circulating atypical cells may be present, highlighting the importance of peripheral blood film review.2The concurrent measurement of acute phase proteins such as fibrinogen can augment the diagnostic interpretation of leukogram data.
When evaluating the leukogram, clinicians should remember that each data set is a snapshot in time of a dynamic system in which the numbers of cells in the circulation reflect a balance between the release of cells from the bone marrow and egress *Contributions to previous editions by Debra Deem Morris and Jed A. Overmann are acknowledged.
of cells into the tissues. Serial monitoring of patients often provides more information than a single time point. It is also important to keep in mind that the numbers of cells trafficking in the circulation may not be reflective of the tissue content of cells; therefore cytology or biopsy of tissues may be necessary to confirm the presence of inflammation or hematopoietic neoplasia.