MCV does indeed refer to the average size of the red cells in a
particular blood sample. MCV is low in iron DEFICIENCY and elevated in
B12 or folate deficiency. During a course of phlebotomy MCV may go up
as the circulation is flooded with new red cells. These new red cells
(known as reticulocytes) are larger than fully mature red cells. This
is entirely normal. It is also possible to have an elevated MCV due to
folate deficiency during phlebotomy, because the very active, red
cell-producing marrow runs out of folate. Patients should generally
take folate every day during a course of phlebotomy to keep the marrow
running (and therefore mobilizing iron) at the maximal rate.

Jerome L. Sullivan, MD, PhD