What doe it mean if rdw lab is low?

All cells same size.: The red blood cell distribution width (RDW) is a measure of variation of red cell volume that is reported as part of a blood count. Usually red blood cells are a standard size (6-8 μm). Certain disorders, however, cause a significant variation in cell size. Low RDW means less variation in size and can be seen when all cells are large (macrocytic anemia) or all cells are small (microcytic anemia).