Anemia is a disorder that results in a decrease in the ability of the blood to carry oxygen. Anemia is itself not a diagnosis but merely a sign of underlying disease. The initial classification of anemia is best accomplished by examination of the data from a hematology analyzer and by an examination of the peripheral blood smear. The physician most commonly classifies anemias initially by the instrument's red cell indicies, especially the mean corpuscular volume (MCV). On newer counters, the red cell distribution width (RDW) or red cell morphology index (RCMI) is another useful measurement. The anemia may be microcytic, normocytic, or macrocytic.