What does "RDW" mean on blood test results?

On blood test results, "RDW" stands for red cell distribution width, which shows the variation in the size of red blood cells, according to the Mayo Medical Laboratories. This measurement is often used to help diagnose specific types of anemia.

A low RDW count is indicative of thalassemia, Mayo Medical Laboratories explains. It is an inherited blood condition, states MedlinePlus. In thalassemia, the red blood cell protein "hemoglobin" is abnormal, and this wipes out many red blood cells.

A high RDW count suggests iron deficiency anemia, Mayo Medical Laboratories advises. In this common type of anemia, the blood has too few red blood cells. Causes include blood loss, poor iron absorption, poor diet and increased need for iron, lists MedlinePlus.