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Red Blood Cell (RBC) Size Variation

Normal RBCs have a diameter of 6 - 8 μm. On a peripheral blood smear, normal RBCs are disc-shaped with a pale-staining central area called the central pallor. When judging red cell size on a blood smear, the classic rule of thumb is to compare them to the nucleus of a small normal lymphocyte. The normal lymphocyte nucleus has an approximate diameter of 8 µm. Of course this method is not foolproof, as red cells that have less than the normal hemoglobin content tend to flatten out more on a slide and may appear larger than they actually are.
MCV measurements are helpful, but 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 homogenous, the cells are microcytic if the MCV is less than 80.0 fL (as seen in the top image), and macrocytic if the MCV is greater than 100.0 fL. However, when there is a mixture of macrocytic and microcytic red cells (as seen in the bottom image), the MCV may be misleading. It may be in the normal range since the MCV is an average. The red cell distribution width (RDW) measurement can be helpful in this case.