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The page below is a sample from the LabCE course Red Blood Cell (RBC) Morphology. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

Learn more about Red Blood Cell (RBC) Morphology (online CE course)

Macrocytes have a diameter of 9-14 µm (1.5 to 2 times larger than normal red cells). Several macrocytes are indicated by the blue arrows in the top image on the right. By comparison, macrocytes are larger than the nucleus of a normal lymphocyte. Macrocytes are frequently oval; two examples are indicated by the red arrows in the top image on the right.
Oval macrocytes are most commonly associated with vitamin B-12 and/or folic acid deficiency. Decreased DNA synthesis causes the nucleus in the developing red cells to mature at a slower than normal rate. Since hemoglobin production is not affected, the mature red cell that is larger than normal is filled with hemoglobin, resulting in cells with little central pallor.
Note that polychromatophilic red cells may also be larger than normal RBCs. However, if stained with a supravital stain, it would be obvious that these cells are reticulocytes. From a Wright-stained smear, they would be reported as polychromatophilic RBCs and not macrocytes. Examples of polychromatophilic RBCs are indicated by the arrows in the bottom image on the right. The cells stain gray-blue with Wright stain.