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Evaluation of the hemogram: What do those numbers mean? (Proceedings)

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Aug 01, 2010

The Importance of a Complete Hemogram

Of all the diagnostic tests available, blood evaluation is one of the single most valuable tools in assessing the general health of the body. Blood, and the nutrients it carries, circulates through every living cell in the body. It stands to reason that it is an incredibly valuable indicator of disease, either local or systemic. The complete blood count (CBC) is the best and most convenient mechanism to detect abnormalities in our patient's the blood. The CBC begins with the quantitative evaluation of erythrocytes, leukocytes and platelets, but it does not end there. It ends with the microscopic examination of the blood film to detect morphological abnormalities that provide valuable insight to various disease conditions.

Quantitative Analysis
Erythrocytes

The red blood cell count (RBC), the packed cell volume (PCV) and the hematocrit all assess the same parameter, red blood cell mass in the body in relation to the volume of plasma. Elevations in these parameters indicate an absolute increase in the number of erythrocytes (erythrocytosis) or a relative decrease in the volume of plasma (hemoconcentration). Another term for erythrocytosis is polycythemia. Polycythemia may be secondary to other clinical conditions such as hypoxia, renal neoplasia, hyperthyroidism or splenic contraction. However, primary polycythemia (polycythemia vera) is a myeloproliferative disease and results from increased production of red cells from the marrow independent of erythropoietin production. Measurement of endogenous erythropoietin levels can give an indication of the etiology. Reduced red blood cell mass (anemia) can either result from a red cell production problem, hemolysis, blood loss, or a combination of the above. PCV alone can provide an accurate evaluation of red blood cell mass, but only with the addition of the RBC can all of the red cell indices be calculated.

The red cell indices, mean cell volume (MCV), mean corpuscular hemoglobin (MCH) and mean cell hemoglobin concentration (MCHC), help us to classify an anemia as regenerative (blood loss or hemolysis) or nonregenerative (production problem), often giving us insight to the etiology. A nonregenerative anemia will typically have a normal MCV (normocytic) and MCHC (normochromic). This can be seen in a number of conditions resulting in decreased red cell production or in per acute blood loss of less than 3 to 5 days duration. The most common cause of a nonregenerative anemia in companion animal medicine is the anemia of chronic inflammatory disease. A regenerative anemia will have an increased MCV (macrocytic) and a decreased MCHC (hypochromic). However, because the indices used to evaluate regeneration are mean values, they will not increase until the population of immature erythrocytes (Reticulocytes) are abundant enough to push the mean values out of the reference range. Therefore, many regenerative anemias will have MCVs and MCHCs within the normal range. The most sensitive way to detect a regenerative response is by performing a reticulocyte count. A regenerative anemia may be seen in conditions resulting in blood loss or hemolysis. A particularly strong regenerative response is seen in animals with hemolytic anemia. Hemolysis results in the most dramatic changes in red cell indicies. An anemia that is the result of chronic hemorrhage and iron deficiency will have a low MCV (microcytic) and low MCHC (hypochormic). In addition, cats with FeLV infection may experience a specific type of anemia arises from disturbances in red cell maturation, resulting in a macrocytic (elevated MCV) and normochromic (normal MCHC) abnormality.