Red cell distribution width (RDW) blood test measures the amount of red blood cell variation in size and volume. Red blood cells are required to carry oxygen from the lungs to every part of the body. High or low range in red blood cell volume or width will indicate a possible problem with a bodily function which may affect the oxygen from getting to various parts of the body. There is a possibility of having a normal RDW along with certain diseases. Normal red blood cells maintain a standard size of 6-8 micrometers in diameter. If the cells are larger, then RDW blood test values will be higher which will result in a nutrient deficiency, anemia or other underlying conditions.