RDW (Red Blood Cell Distribution Width): RDW measures the variation in size of your red blood cells. A healthy blood sample has red blood cells that are mostly the same size, while a wider variation can indicate certain types of anemia or other blood conditions. Normal RDW means your red blood cells are fairly uniform in size. High RDW may suggest iron deficiency, vitamin B12 or folate deficiency, or recent blood loss, where new red blood cells are a different size than older ones. Low RDW is uncommon and usually not a concern.