MACROCYTOSIS - Macrocytosis is red blood cells is defined as a red cell mean corpuscular volume (MCV) of greater than 100 fl. Macrocytosis is present in 1 to 4 % of the adult population. The most common cause is alcoholism. Other causes include: nutritional deficiencies (B12 and folate), chemotherapy, drug side effects, hemolysis, liver dysfunction, myelodysplasia, and hypothyroidism.