One way of classifying anemia is by morphology. The usual starting point is determining the average size of the patient's red blood cells (RBCs). The average cell size of normal erythrocytes is 6-8 μm in diameter. We can classify all anemias as normocytic (normal size), microcytic (smaller than normal), or macrocytic (larger than normal) based on this reference range.
Macrocytic means "large cell" and thus the average size of the red blood cells would be greater than 8 μm. Making such a determination requires an ocular micrometer to measure the RBCs in a Wright-stained blood smear. A way of estimating cell size without an ocular micrometer is by comparing the diameter of the RBC to that of a lymphocyte nucleus. The nucleus of a normal small lymphocyte is approximately 8 μm, so any RBC larger than this could be called a macrocyte. The top image to the right points to apparent macrocytes as compared to the lymphocyte nucleus. The second image depicts microcytes, RBCs smaller than the lymphocyte nucleus, as seen in microcytic anemias.
Another way to classify morphology is by using the red blood cell index known as mean corpuscular volume (MCV). The reference range for MCV is 80–100 femtoliters (fL), although this range can vary based on individual labs and instruments. The MCV in macrocytic anemias is greater than 100 fL, or whatever the upper limit is in your laboratory.
Seeing macrocytes doesn't necessarily mean that the patient has macrocytic anemia. To be classified as anemia, the patient's hemoglobin must be below the reference range for their group. Therefore, a person with macrocytic anemia would have an MCV above 100 fL and hemoglobin below the reference range for their demographic.
