Red blood cell distribution width (RDW) is a measure of the variability in size among red blood cells (erythrocytes) in the bloodstream. It represents the anisocytosis, which is the difference in size between younger and older red blood cells. The RDW is commonly included in a complete blood count (CBC) as a measure of the body's production of red blood cells. A high RDW can indicate that there is a problem with red blood cell production, such as anaemia, or it can result from a destruction of red blood cells, which may be due to diseases like sickle cell disease. On the other hand, a low RDW is usually within normal limits and indicates a consistent size distribution among red blood cells.