Red blood cell distribution width (RDW) indicates whether your red blood cells are all the same size, or different sizes or shapes. Normally cells are fairly uniform both in size and in shape, but some blood disorders may cause your red blood cells to form in abnormal sizes. This test measures the difference between the largest and the smallest red blood cell.
What might a high result mean?
An elevated RDW can indicate that you have macrocytic anaemia, where your red blood cells are larger than they should be. This can be caused by not having enough vitamin B12 or folate in your diet, or by having a condition which prevents you from absorbing enough vitamins, such as Crohn's disease or pernicious anaemia.
An elevated RDW may also indicate that you have microcytic anaemia, where your red blood cells are smaller than they should be. This is generally caused by iron deficiency, which can be due to a lack of dietary iron, gastrointestinal bleeds and pregnancy.