Macrocytic anemia is when the red blood cell size is larger than normal, i.e.an MCV of >100. B12 deficiency is one of the common causes of macrocytic anemia. Microcytic anemia is when the red blood cells are smaller than normal, i.e. the MCV <80. When iron deficiency is diagnosed as the cause of a petientâs microcytic anemia, the most important task is to discover why the patient is iron deficient. The cause can be either poor intake or absorption of iron from the diet, or loss of iron in blood cells lost from apparent or occult bleeding.