What is an RDW Level?

The RDW stands for the red cell distribution width. This test is a measurement of how much each red blood cell varies in size. The RDW is part of a blood test, the complete blood count or CBC, that measures many other components of the blood and blood products.

What the RDW Means

• Anemia can be the cause of a RDW that falls out of the normal range.

  The RDW tells the variability of the size of the red blood cells. A low count means that the red cells do not vary a lot in size whereas a high count means the cells vary a lot with very small and very large red blood cells. A finding that is either low or high would indicate the need for further testing to identify the cause of the results.

What is Normal

• There is normally some variation in the size of red cells, and a normal RDW will fall between 10.2 percent to 14.5 percent. This figure will be slightly different between labs but the normal range will be given with the test report.

A Low RDW

• A low RDW means that all of the red cells are about the same size. This can be caused by macrocytic anemia where there are very few red cells but they are all very large. It can also be caused in microcytic anemia where there are few red cells but they are all very small.

High RDW

• A high RDW indicates a wide variation in the size of the cells, from very small to very large. This can be caused by conditions such as a deficiency in vitamin B12 or folic acid. In addition, it could point to iron deficient anemia or to thalassemia intermedia.