Something important to keep in mind is that a patient could have an increased MCV, but it is not caused by ineffective erythropoiesis (that is, it is neither a megaloblastic nor non-megaloblastic macrocytic anemia). This situation is caused by the bone marrow compensating for anemia by releasing many reticulocytes. Reticulocytes are larger than fully mature erythrocytes, and if a high enough reticulocyte count exists, it could raise the MCV slightly.
It is usually easy to differentiate this situation from true macrocytic anemia by looking at the reticulocyte count or examining a Wright-stained blood smear for polychromasia. Reticulocytes will appear polychromatic or slightly blue staining on a smear. Also, the MCV will only be slightly elevated, not profoundly elevated.
The image to the right depicts polychromasia (i.e., reticulocytes).
