This article is from the Interpretation Of Lab Test Profiles, by Ed Uthman email@example.com with numerous contributions by others.
The red cell distribution width is a numerical expression which
correlates with the degree of anisocytosis (variation in volume
of the population of red cells). Some investigators feel that it
is useful in differentiating thalassemia from iron deficiency
anemia, but its use in this regard is far from universal
acceptance. The RDW may also be useful in monitoring the results
of hematinic therapy for iron-deficiency or megaloblastic
anemias. As the patient's new, normally-sized cells are
produced, the RDW initially increases, but then decreases as the
normal cell population gains the majority.