A red cell distribution width (RDW) test is a measurement of the range in the volume and

size of your red blood cells (erythrocytes). Red blood cells move oxygen from your lungs to

every cell in your body. Your cells need oxygen to grow, reproduce, and stay healthy. If your

red blood cells are larger than normal, it could indicate a medical problem.If your RDW is

too high, it could be an indication of a nutrient deficiency, such as a deficiency of iron,

folate, or vitamin B-12.