The Full form of RDW is Red Blood Cell Distribution Width. RDW blood test measures the amount of red blood cell variation in volume and size. Human beings need red blood cells to carry oxygen from lungs to every part of the body. Anything outside of the normal range in red blood cell width or volume indicates a possible problem with bodily function that in turn may affect oxygen getting to various parts of the body. However, with certain diseases, a person may still have a normal RDW. Normal RBCs (red blood cells) maintain a standard size of 6 to 8 micrometers (µm) in diameter. If cells are larger, the RDW blood test values will have higher numbers. This means a person may have anemia, a nutrient deficiency, or other underlying condition. The RDW test is used to help diagnose types of anemia and other medical conditions including diabetes mellitus, thalassemias, which are inherited blood disorders that can cause severe anemia, heart and liver disease, cancer etc. This test is commonly performed as a part of a CBC (complete blood count). The CBC determines the types and number of blood cells and various other characteristics of your blood, such as measurements of platelets, red blood cells, and white blood cells.