What Does RDW Mean on a Blood Test?

What it means

RDW stands for red cell distribution width. In a typical blood sample, red blood cells are mostly similar in size. RDW measures how much they differ from each other. A higher RDW means there is a wider mix of small and large cells; a lower RDW means they are more uniform.

RDW is part of the red cell section of a complete blood count (CBC), next to measures such as hemoglobin and MCV. The lab includes it because it helps doctors think about causes when other red cell results are unusual. For example, when iron, vitamin B12 or folate run low, new red cells may come out a different size from older ones, and RDW can rise.

RDW is different from MCV. MCV is the average size of red cells, while RDW is how spread out the sizes are, so two people can share the same MCV but have very different RDW results. Some labs print RDW-CV, a percentage, and others RDW-SD, in femtolitres.

Where you may see it

• The red cell section of a CBC, near MCV, MCH and MCHC
• Patient portal results listed as "RDW-CV" or "RDW-SD"
• Anemia workup letters from a primary care doctor or hematologist