Anisocytosis
means that the red cells are of unequal size. It is a feature of many anemias,
and other blood conditions, and does not have much diagnostic value. The
'red cell distribution width' (RDW) is a quantitative measure of the degree
of anisocytosis. The RDW is useful in the differential diagnosis of microcytic
anemia. Most cases of iron deficiency have a raised RDW, and most cases
of thalassemia trait have a normal RDW.