Macrocytosis is the frequent occurrence of macrocytes. These are erythrocytes with a diameter of more than 9 µm and a MCV < fl. Oval macrocytes are typically found in megaloblastic anemia.
Macrocytosis occurs when abnormal erythropoiesis is present as in alcohol consumption, liver disease, during use of cytotoxic (e.g. azathioprine, hydroxyurea) or virostatic drugs (e.g. zidovudine), in megaloblastic anemia through a vitamin B12 and/or folic acid deficiency as well as in myelodysplastic syndromes.