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MCV and Anisocytosis

Mean cell (or corpuscular) volume (MCV) may be helpful in determining red cell size; however, it is important to remember that the MCV is the average size of the red cells in the sample. When the red cells are all similar in size, or homogeneous, it is fair to assume the cells are microcytic if the MCV is less than 80.0 fL, and macrocytic if the MCV is greater than 100.0 fL. However, when there is a mixture of macrocytic and microcytic red cells, the MCV may be misleading. It may be in the normal range since the MCV is an average.
Anisocytosis is a general term used to describe variation in size of the red cell population present on a blood smear. Notice that normal, small, and large cells can be seen in this field. Since several populations of cells are present, this abnormality will not be reflected in the MCV value, which is the average size of all the red cells that are counted. However, anisocytosis will affect the red blood cell distribution width (RDW), which is a measure of red cell size variation. As the severity of an anemia increases, the amount of significant anisocytosis present may also increase.