Erythrocyte distribution width (RDW) is a blood test that measures the variability in the size and volume of red blood cells. Red blood cells or erythrocytes are small, round, biconcave cells that contain hemoglobin which carries the oxygen from the lungs to tissues and carbon dioxide back from tissues to the lungs. 

The RDW test is normally ordered as part of a complete blood count (CBC) panel, which is used to help doctors assess your overall health and diagnose a wide range of medical conditions. A CBC panel can help doctors detect infections, anemia, leukemia, and other blood disorders. When the variation of red blood cell size is higher, meaning there is a large difference in size between the smallest and largest red blood cell, then the RDW is elevated. 

A high RDW result can be caused by a variety of conditions. It may indicate that a person has anemia from a deficiency in iron or vitamin B12. In some cases, an abnormal RDW may be due to a chronic health condition, such as kidney disease, liver disease, diabetes, or an autoimmune disorder, such as lupus or rheumatoid arthritis. Some cancers, especially colorectal cancer can cause your RDW levels to rise. 

On the other hand, a low RDW may indicate that the red blood cells in a sample of blood are all similar in size and shape. It can indicate that a person has polycythemia vera, a rare blood disorder in which the body produces too many red blood cells. It can also indicate the presence of certain medical conditions, such as dehydration or thalassemia.

Overall, the RDW blood test is a useful tool for evaluating a person's red blood cells and identifying potential health issues. However, even if your RDW levels are in the normal range, there is still a possibility that you are facing a medical condition that requires treatment.