Medically reviewed by Gagandeep Brar, MD The red cell distribution width (RDW) blood test measures differences in the volume and size of your red blood cells. Red blood cells help carry oxygen from ...

In this case, the megaloblastic anemias usually have a high RDW, while non-megaloblastic have a low RDW, helping to make the distinction. RDW can also be very helpful in mixed anemias. For example ...