The mean cell volume indicates the volume of the "average" red cell in a sample. It is expressed in femtoliters (fl; 10^-15 liters). Traditionally, MCV was a calculated parameter, derived by using the following formula:

Present-day automated hematology analyzers, such as the Advia, provide a more accurate, direct measure of MCV. With impedance analyzers (e.g. Heska bench top analyzers), the MCV is measured by

Histogram and red cell parameters from a normal canine blood sample.

averaging the amplitude of the pulses created as the cells pass the aperture of the counter. (See discussion of impedence counters under RBC section). The instrument also "channelizes" the impulses, segregating them into channels representing ranges of cell size. This data is then assembled into a cell size histogram, such as shown at right. The histogram provides additional useful information about the characteristics of the red cell population.

The Advia also provides a histogram, although red blood cell volume is determined from the amount of low angle light scatter as the cells pass through a laser beam. This instrument also plots the cell volume and hemoglobin concentration together in a volume/hemoglobin (V/HC) cytogram. In this cytogram, hemoglobin concentration is plotted on the X-axis and cell volume is plotted on the Y-axis. This cytogram provides a lot of useful information as the red cells are separated into grids of based on red cell volume and hemoglobin concentration (e.g. macrocytic hypochromic). Normal red blood cells with normal volume (normocytic) and hemoglobin concentration (normochromic) are found in the middle box (as shown in this cytogram from a dog above).

Red cell populations with MCV above reference range are termed macrocytic. Common cause of macrocytosis are reticulocytosis and myelodysplastic syndrome (MDS) in cats. Some individuals of the Toy and Miniature Poodle breed have larger red cells compared to other breeds due to a congenital dyserythropoiesis which is otherwise asymptomatic. Red cell populations with MCV below reference range are termed microcytic. Common causes of microcytosis are iron deficiency anemia and chronic liver disease in dogs and cats (usually portosystemic shunting). Akita dogs, as a breed, have MCV values below that of other breeds in the absence of any disease process.

Abnormal cytograms with the Advia can indicate underlying hematologic changes. For example, a regenerative response can be seen as a population of macrocytic hypochromic cells extending off the normal red cell population. Agglutination can be seen as a macrocytic normochromic cluster of cells that is discrete from the normal red cell population.

A. Normal RBC V/H cytogram in a dog B. Dog with polychromasia: the polychromatophils form a discrete cluster (arrow) that extends off the normal red cell population (arrowhead) and is macrocytic and hypochromic.	Agglutinated cells (arrow) in a horse with IHA form a distinct cluster (macrocytic normo-chromic) above the normal red cells (arrowhead)

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Last Updated: June 2000